In case there are others on here wondering - Nikon released an updated firmware for the D850 today - see here: Nikon | Download center | D850 Firmware
I tested it with a NEF after upgrading, and v5 of PhotoLab is fine with it - works perfectly.
Thanks to you and @mwsilvers for pointing this out. I already have the immediately previous version and don’t have any problems that the latest is said to solve, but I will update anyway
When I looked yesterday, the new version was 1.21. Today it’s 1.3, which I have just installed and PL6 seems to handle the file fine.