Ramsters show a vertical range when placing them, but will never deliberately attack an air unit (though they will damage air units that bumble into their line of fire when shooting at other units). May be the underlying cause of this bug, due to Betsy being partially aerial.
Either the vertical range shown is incorrect, or the Ramster's inability to fire on air units is incorrect.
Note when reproducing: You must keep other ground enemies outside of the Ramster's range, and ensure that the flier does not approach ground level - too low, and the Ramster may 'confuse' it for a ground unit and fire anyway. Make sure that the Ramster must use its alleged vertical range.
Screenshot - note that the Ramster has a vertical range line.