When placing a repeating command block in the spawn area (in the chunks constantly generated), at some point in time, the command block will stop updating the command inside.
When it happens, it will happen for all repeating command block in the area as every near command block will show they last updated their command at the same time.
To unlock them, you have to change some parameters in the command block, press done then return in the command block interface to revert the change and press done again.
Taking the command block in hand, keeping the nbt information, and destroying and replacing the block will not break the bug.
Bug occurred several times on a multiplayer server on 1.14.1 with approximately always 4 players connected. The bug didn't seem to happen in 1.14 but it may have. Bug still occurs in 1.14.4.