This post explains why there is not a version 2.9 Andriod app on the Play Store.
TL;DR - By the time the app has been updated and modified to meet Google's Play Store publising requirements, Google have further changed those publishing requirements.
One point made in the post is, "... therefore there will not be a 2.9 version for Google Play." This suggests that later versions will be on the Play Store when Joplin can catch up with Google's ever-changing requirements.
So this appears to be a temporary situation.