I guess I'm another lucky cMP user with 11.4 running perfectly smooth since update a few days ago and no issues with booting whatsoever after ~20 reboots/shutdowns also while switching between MacOS and Win10.
My setup is as follows in case I can help you somehow solve the mystery:
-Mac Pro 5,1 (2012) single cpu W3680, 24GB of RAM
-Bootrom appears in system info as 9144.0.6.7.0, but i'm guessing that's some sort of spoof/error?
-Running OpenCore 0.6.7, Martin Lo's package with just HWAccel and MP 7,1 Board ID added.
Hardware:
-SATA SSD in Bay1 with OC and Big Sur - Crucial MX500 with TRIM enabled
-HDD in Bay2 with just data formatted in APFS
-NVMe Kingston KC1000 with Win10 in a generic ICY BOX adapter in PCIe slot 4
(-RME HDSPe AIO audio interface in slot 3 - probably not relevant)
-Radeon RX5600 XT in slot 1, display connected via DP (this caused me some issues with booting in the past so might be worth mentioning)
Updated directly from 11.2.3, the update preparation took forever - about 30mins i'd say, but then after a few reboots (which i believe were always the case while updating Big Sur, not only with 11.3/11.4) the system loaded normally and everything seems to be working fine ever since.