1.16.4 command execution blocks have a chance to fail detection if entity stands on certain coordinates, when I was in the following coordinates, it won't work as with the "Always Active" redstone activation, but it was fixed later on if you destroy a command block and replace it and paste the same command or control + middle click command and destroy and place. It was annoying to mapmakers. Same as me. It will probely happen if you use WorldEdit and move structure around.

Affect detection commands:

/execute if entity @p[x=#,y=#,z=#,distance=..#]

/execute if entity @p[x=#,y=#,z=#,dx=#,dy=#,dz=#]

NOTE

If you think I was faking the bug, I was screenshot around 3.531 seconds away from functioning text. I also break the walls to show you I didn't break the command blocks.