I was getting notifications daily on my pinephone in the “Discover” app that there were upgrades available, and I was upgrading them, so I thought it was working. Last night, I received a notification that a new kernel was available - I think it said something about “6/7/21” instead of “5/12/21”, so I upgraded to that one. It said it was successful. But I did a
uname -a
and I’m still showing "Linux plasma-mobile 5.12.9-1-MAJARO-ARM #1 SMP
For switching branches you would also have to edit /etc/pacman-mirrors.conf, but I’m guessing you did that. The kernel version numbering has nothing to do with the date, 5.12.9 doesn’t relate to 5/12/21 if that’s what you mean. I believe the kernel that shipped with the last Plasma beta (stable) is 5.12.8, so your’s should be newer.
That’s the file where you change the branch from arm-stable to arm-unstable. This is also possible with the pacman-mirrors command (sudo pacman-mirrors --api --set-branch arm-unstable; more in the wiki), maybe you used that one. In either case you can check in that file which branch you are currently on.