since the Aug 20 update my Samsung S4 tab is not working properly , heart rate constantly dropping out , the option to use ERG mode is often not available , when this happens the app becomes unusable , to fix this the app has to reinstalled , if that does not work , I then use either an old LG g6 phone or Samsung A50 phone both work well ( but a little small ) , is zwift no longer supporting the Samsung s4 TAB ???
It would be odd that they drop a paltform so soon after recommending it Android Supported Devices (UPDATED: Dec 22, 2019)
Your problems sound like a lot of other problems people are having since the August update. Sorry, cant help but do feel your pain!