Yes, it is strange. I did manage to boot TWRP on FP3 even though the fastboot mode says the device is locked. I need to dig out the instructions I was following, as I wasn’t taking notes. I made the upgrade to Q back when stable builds were not yet being published, so I ended up on the dev branch then. I honestly don’t remember the steps for that either, regarding unlocking and relocking, but I know I had to wipe data.