The Bug
Beacons built and activated in 1.17.1 or earlier turn off, despite still being set up perfectly, but can be reactivated by breaking and replacing some of the blocks.
Reproduce
Make a world in 1.17.1
Build a powered beacon
Leave and open the same world in 21w43a
Observed Result
The Beacon was deactivated
Expected Result
The Beacon would stay activated
Is this still an issue in 21w44a? Please try loading directly from 1.17.1 to 21w44a; for worlds loaded in 21w43a erasing the caches might solve the issue (Select world in the menu, press Edit, press Optimize World, check Erased cached data checkbox, proceed)