The problem is that Android and other mobile OSes added so many restrictions on what one can do for the sake of security (even though there are still more pressing security issues present) that many frameworks do not keep up with adding workarounds or using the new APIs.
Joplin is using react-native on mobile. Background processes are not easy to accomplish with that framework. Additionally, Android freezes apps in the background. Thus even if we had background sync, the OS would kill it.
Please thank Google and Apple for making development for mobile devices a living hell.
Android 10+ (10, 11, 12) on Google Pixel (1 and 5) does it as well, but there's on option to turn that off. But I remember experiencing issues even though it was turned off.
e.g. I have turned it off for DAVx5, K9-mail, and Signal, but sometimes I still receive mails only when I take the phone in my hands. (I use my own mail server with push notifications.)