That’s really odd, as the controller apps are merely windows, showing what is happening on the Sonos system itself, there’s very little going on on the controllers themselves. Which suggests some potential connection / network issues between those two devices and your router. Do you have a mesh network, possibly, and the iPhone and IPad are connected to different nodes?
I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, from both devices, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.