This happened years ago with Paul Scholes rejoining Utd as a player after a season coaching. It's happened again today with Ben Foster, the "Cycling GK", coming out of retirement to join Wrexham. I'm sure it's happened many times in-between, too. This happens too frequently NOT to be included in FM.
I should say I haven't played any FM version since 21 so I'm unaware if this has been implemented already. I'm rocking a long-term save and really enjoying the 21 experience. I've not seen it happen on this version.