I received the following update from another platform I'm on regarding this issue. I have not verified this information, but believe it to be true:
It has nearly been ten months with no fix in sight still for the issue. However, Microsoft finally has another update on it thanks to a response from the Microsoft OneDrive product group, and it is supposedly due to a backend migration issue that the tech giant is currently still performing.
However, it does not give any specific clue as to when this migration phase will be completed, though Microsoft is trying to "speed things up":
The message reads:
"This is related to a backend migration we are currently doing. Unfortunately, we do not fully support syncing shared folders when one user has migrated and the other has not . We are doing everything possible to speed things up so that the issue can be resolved as quickly as possible for users of the feature. When the accounts are migrated, there is an automatic logic that ensures that they return to a synced folder within a few days."