I had hoped the new 10.15.6 would solve the "disconnecting thunderbolt monitor after putting MBP to sleep" issue, but it hasn't. I just got a hard crash after opening the lid back up upon moving from workstation to lap. 16" MBP, i9. Ugh.
For what it's worth, I had the same crash without taking it off the monitor. I just woke it from sleep this morning, and crash reports has a WindowServer crash from waking up. *sigh*