don't know about the version issue and can't test it but I'd be very surprised.
Start by creating a text box without the "show" condition expression.
in the tex box put the expression:
now start selecting on your [Projetc Type] field. What happens to the text box? This should display the selected value of [project Type] when one and only one value has been selected. Is this the case? If not there is a typo.
The idea is to test the result of the above expression against the string 'Maintenance' and show the textbox if this condition is met.
try again - I tested it her and it works (v8.5).
by the way Sivasubramanian's solution woul ddo the job too.