I have a simple calculation in a Calculate Field tool in modelbuilder. %Field Name% is a variable that references an actual field that is in the attribute table.

This is what the results look like:

I have been using inline variable substitution regularly and have never seen anything like this. If I double-click the field from the drop-down, instead of using '!%Field Name%!', it works as desired.
Does anyone know what is going on here and how to correct it?
Thank you,
Randy McGregor