Re: About "Sync Devices" Feature
Frankly, I don't know. This whole sync code is so complex and impossible to maintain, that I'm slowly depreciating and removing it. The first step of this removal was "auto-sync", it relied on Google Firebase servers that were not reliable of sending timely notifications of changes form one device to another. I'll probably remove "sync devices" completely soon.
The original idea was that whatever you added on one device, will be also copied to another, what you deleted will be also deleted from the other. If status changed, e.g. you read further in an ebook, that progress would also be changed on the other device etc. Sounds simple but gets infinitely complex and error-prone when you go into the gory details.