The first time this happened to me I sent in a ticket, and this was support's response:
[pj],
There is a bug involved here, but it's not what you think and it has nothing to do with end-of-season injury status. The process for healing during the offseason is working as intended.
The bug lies in the recovery cycle. In this case, your player "healed" more than he should have (beyond his projection). So when the normal development cycle period began, he started falling as a result.
We will be investigating the cause of this inadvertent boost during his earlier recovery cycle. We've seen it reported before, but have not yet tracked down.