I used to run my 15" MBP with 21.5" iMac in Target Display Mode and everything was fine for a while. But as of a couple recent macOS upgrades, every time I disconnect, the iMac never restores to it's previous state. The screen is black. Once in a while, after 10 minutes or longer, it would come back. But more often than not, I have to power-cycle my iMac.

I have tried every way possible to connect/disconnect. Different port combos, Apple genuine cable, unplugging Thunderbolt physically, Cmd+F2. All of this used to work, but not anymore.

It's really annoying because even if I let my MBP go to hibernate/sleep/screen-off, I end up having to power-cycle my iMac.

Any suggestions would really be appreciated. Is anyone having this experience, as it doesn't seem common?

I have not found anyone complaining about this exact problem.