Email notification when spicific field changed missing user fields

Is [Assigned To] a lookup field?

No this is a member if the Tasks table

A crazy thing to try (but I would do if this wuz me) would be to make a formula text field of

UserToEmail ([Assigned to])

 and see if that field appears.

Crazy thing that worked =D

Sounds to me like a bug.  I had another field giving me odd behavior lately.  I had to rebuild the field.  Try building a new [Assigned To] user field and see if it shows up on your notification criteria.

:) so  sometimes with Quick Base there are weird bugs or just stuff that you cannot figure out and we find a work-a-round and just move on.  Chalk that one up to the world of the unexplained" and move on.