That has not been the case with me. I have just booted into 6.4.2b (0623) successfully, re-installed this week's ROM (20-11-12) and Magisk, and now just rebooted successfully back into the system (my phone). There has been no problem with it. I am keeping the new TWRP that was installed with this week's ROM, and just booting into the other TWRP:
fastboot boot recovery-TWRP-3.4.2B-0623-XIAOMI10-CN-wzsx150.img
For the moment, I intend to keep working like that. Before I update with the new ROM, I always take a backup anyway.