As a matter of fact, I did terminate the contract of a reserve team coach at one point during my save. However, I lost control of all staff responsibilities long after that - after one or two in-game months, maybe even more. If that is what triggers the initial defect, then why would it take so long? I'm genuinely intrigued.
To test things out, I terminated my own contract after losing control of staff responsibilities, and then immediately added myself back as manager by using the in-game editor. This only solved the issue temporarily. After adding myself back as manager, I went on holiday for a month. By the time I came back, I had lost control of staff responsibilities once again - this time, without touching staff.
Very, very strange.
For other users, this is the issue that I raised:
Let's hope we get a fix of some kind. I would hate to lose all my progress on this save.