Just did an update via the Avast app, (I don't know why Google Play did not do that app update for me) to V24.3.0-1004091-eca47db16.
The UI is a bit different. I just went to Webshield page which displays "Your browsing is secured" etc.
That page is now in dark mode, not light mode, so is the same theme as rest of the UI.
The small backarrow top left now works as it should, as do the off screen "Recents" and "Back" switches.
I can only assume the UI of my previous version must have been faulty. I will monitor the behaviour of this version and report back if the fault behaviour returns.