There were quite a few people mentioning issues with topics not being marked read on Android.
We have done some root cause analysis and found that the page visibility api changed the way it operates in Android Chrome 77.
It was discussed in brief here:
There is a fancy graph that explains the new lifecycle here:
The symptoms of the issue is.
You use your android
You flip to do something else for 5-10 minutes
You flip back to Discourse and nothing is marked read anymore
I made a fix for this here:
In “theory” it should resolve the issue.
This fix will be deployed on meta shortly, you will have to do a full reload to get it applied.