Some suggestions…

Perhaps try fully closing (sliding off screen) the iPad controller app - leave it closed for a minute or so and see if they return. If still no joy then perhaps try these next two things (not in any particular order):

• Disable/Enable Personalisation Services in "Settings/Data & Privacy’ - note you will lose the recently played list on the My Sonos Tab if choosing to do this.
• Reset the App in "Settings/App Preferences” ...on reopening the App, skip through the introductory pages and choose the option to join the ‘existing Sonos System’ - do not create a new system.

Try SonoPad from the App Store. The official iOS Sonos app has a bug that causes playlists to randomly vanish on some devices.

I have the same problem and the Sonos app takes longer time to load when I am starting up the app.
This must be at Sonos problem and not a “local” problem.

Worth trying a reset of the controller?

I am not so sure ;-)

OK I concede it's not impossible

 I would try (with no guarantee of success)

Power off Sonos speakers

Reset the iPad / iPhone Sonos controller 

Power speakers back on

'Connect to existing system' in the app. 

(Are system and controller on latest firmware?)

And to be honest, I have had the SonoPad app. on my controller for years and I now use the SonoPad app more and more, because it responds faster than Sonos own app., which has started to take too long time to reload. Therefore I can also recommend the SonoPad app.

I have repeatedly tried these steps, when at it is best, the problem is solved for 30 minutes and then the error reappears when the app updates.

I think because not everyone experiences this error it could depend on which Sonos products/hardware versions you have in your Sonos set up. And as “controlav” also writes I also think this is  a periodic bug in the app. and I also think Sonos allready know about this bug.

Maybe you’re hitting some kind of limit with your playlists? - are they perhaps many in number and have lots of tracks? I only ask because I’ve not personally ever seen any of my playlists disappear in the S2 App - the only thing I have an intermittent issue with, is the ‘recently played’ section of the ‘My Sonos’ tab, which can sometimes not populate, but everything else has been working okay. 

I’m just not sure what the limitations are for the number/size of the playlists allowed in the App. Perhaps someone else here can shed some light on that for you, if only to help eliminate that as being the cause.

I just again did all the resetting things you suggested.

You ask if it is due to many playlists, the answer is no, we are talking about  only one playlist with 30 tracks.

And when the playlist appears on My Sonos, it is many times without the front art on the start tile button.

You are also mention the problem with the periodic missing last played tiles, this problem I have too.

And again the app also takes longer time than normal to load.

And this is not because off WiFi interference in my home, but it could be WiFi interference on the US Sonos server ;-)

But again I am convinced that Sonos is aware of this issue.

There is no ‘Sonos server’, other than for Sonos HD radio, and for updates to the controller. All of your data is local to your speakers, and not stored on a Sonos server at all. 

I was just trying to be funny, sorry. But now that you mention Sonos radio, I mostly listen to radio via my local national radio station which has its own Sonos App. It works fine and fast and with high speed streaming / “HD sound quality”. When I occasionally want to listen to a radio station via Sonos Radio, the app takes long time to connect to Sonos Radio, just for information.

A new thread about this problem but this time on Android.  
 

One more thread with same problem as in this thread.

Not sure if this will help others but i have the Sonos App (S2) on both my iPhone X and Ipad Pro. Every now and again i cannot see my Sonos created playlists in one or both apps. All i do is go to my main PC which has the Sonos App and reset controller. I reconnect to an existing system and both phone and ipad then refresh and the playlists appear again

Same here.  Can see playlists on Android but not my iphone or ipad.  Thought it might have been fixed with yesterdays update.  Then a few minutes later they reappear

Hi everyone, thanks for raising this with the community.

Our engineers are aware of this issue and are working on getting it resolved - no timescale for a fix that I can share right now, however for the time being the workarounds are force quitting & re-opening the app, or starting playback from a different part of the app such as browsing the service or searching for the playlist directly. This issue is only known to affect devices running iOS (iPhone, iPad, iPod) and not Android or desktop controllers.

Same problem here. Rocket science? I think not. 

Great. Please get a job with Sonos and fix the code. Thanks. 

Use SonoPad/SonoPhone/Phonos Plus to play your playlists instead, while you wait for Sonos to fix their app.

I would never have discovered SonoPhone had I not been driven to complain on this forum. It’s good to have options! Using it now!