... got some TVS's that I was assured would be right for my application.
... part number for the TVS's I purchased is: SA24A+ TVS DIODE, 500W, 24V, UNIDIR, DO-204AC
Where did you get the advice on component selection?
1) You need a bi-directional (not UNIDIR) TVS. Unidirectional is generally for DC operation.
2) 24V is too low of a clamping voltage for a RS1 18.5V AC output. The peak voltage of an 18.5V AC output is over 25V. So a 24V TVS would be engaged on each AC line cycle in "normal" operation.
See this OGR post for some suitable bi-directional TVS parts.
I'm surprised you didn't see the dreaded magic smoke wafting from the TVS.