I understand what you said. I'm saying that most of the time, what happens is actually the opposite of what you said.
and it keeps on asking me to update proton mail to 3.0.16, and this version is not on the play store.
Version 3.0.16 has been on the Play Store for quite some time, I've got it updated through Aurora Store. Since July 25th to be precise. So it took them about 17 days to publish it to GitHub, after pushing it to Play Store.
The Play Store doesn't always push an update at the same time to all users. So instead, they have a Staged Rollout. Here's an example from this article, it's not related specifically to Proton.
No, at least not on Android.