I’ve not personally noticed such issues with the Sonos Home Theatre setups (with Sub) I have here.

My first thoughts were… "Night Mode" enabled, perhaps? 

You really need to provide more detail, ideally with some examples of the audio sources you’re playing, so others here with the same/similar setup can perhaps try to replicate and then comment.

There are lots of things where Sub frequencies might be less than expected, not least a faulty sub, it’s position in the room, the playing audio, EQ settings etc; and such things as mentioned in this link:

https://hometheateracademy.com/subwoofer-sounds-weak/

However, starting with some examples perhaps of what you’re playing would be good and describing the setup in more detail. Maybe even post a photo of the listening environment and type/position of your devices.

If no luck with finding others who have your issue, you could always take the option of submitting a system diagnostic and note or post it’s reference back here and then contact/chat with the Sonos Support Staff via this LINK and see if they can see any issues with any of your devices.