This bug was observed in Bedrock versions 1.19.83 through to 1.20.4.

Villagers who have professions and whose profession has been locked by trading will lose connection to their jobsite block. They will sometimes reconnect to their jobsite block if it is undisturbed. However, if there are other villagers around, then during this period the unemployed villager next in line for a profession can connect to the jobsite block. This can then trap the profession-locked villager in a state where they are not able to work or refresh trades.  An extension of this is that if a profession-locked villagers' jobsite block is destroyed, they cannot target a replacement jobsite block in a new position, and sometimes even in the original position.

The only known in-game solution is to kill the affected villagers, which is undesirable.

Additional details:
I have an "organic" village: The jobsite blocks are in player-built houses with fenced in yards, and the villagers are not constrained to small trading booths, but rather are able to wander in and around their homes, usually contained within the yard. I have found that they routinely lose connection with their jobsite, in spite of there being a pathfinding-compatible route to their jobsite at all times. As a result, their connection is routinely broken, disrupting my ability to maintain the village in a play-state that I believe closely approximates Mojang's intended relationship with the villagers, rather than containing them in 1x1 closets immediately next to their jobsite at all times.

Interactions:
This bug interacts with another bug  MCPE-176342 , in which villagers that have a profession but who have not been traded with will have their profession made permanent if the player logs out of their Realm, and occasionally when the player leaves render range.  These combined bugs lead to a profession propagating and being made permanent across all available villagers over a period of play, filling a village with fixed-employment villagers who have no jobsite.