Interesting. I was able to do this by moving to a cell, just hitting 'Enter' to give this:
Screenshot 2021-10-14 at 14.31.47.png
and then moving back up to the first cell and typing what you would expect:
Screenshot 2021-10-14 at 14.32.06.png
Hitting 'Enter' again then assigns the required contents of the cell correctly:
Screenshot 2021-10-14 at 14.32.18.png
(Using JMP 16 on a Mac).
I would suggest sending an email to [email protected] referencing this thread.