[image: 0fef8fb6-ab27-4b97-9278-cfdfe0604257.jpg] Believe I've found a workaround to this annoying bug!!! It seems to have to do with the altimter qnh/std button. I've been flying in areas mostly with very low transition altitudes (3,000 ft or so) and have been putting this button into STD. Don't do this. If you spin the altitude window up with it in standard, it will give you the invalid cruise message, and you can't engage profile. The solution I've found is as follows:
Just leave the button in QNH and manually set the altimeter
If you do switch to STD, but level off at an intermediate altitude before cruise, you must reselect QNH, then on the PROG page of the FMC, change the cruise altitude to the altitude currently selected in the autopilot (i.e. 12,000). I was able to successfully reengage PROF. I could then switch back to STD and it would stay.
Curious to hear if this helps anybody else.