Not a FOMO mechanic by any means... That said...
Last year I put a new 383 stroker motor in my '97 Silverado, pulling out the original 350 (good condition, just wanted a bigger motor). I changed the Torque Converter & re-built the transmission in the process. A lot of things have to be disconnected to get at the torque converter. It is not outside the realm of possibility that an oil filter / cooler line / etc was loosened or disconnected in the process of changing the TC. In fact, it is pretty darn likely.
Find out where the loss of fluid came from. If it was something disturbed in the process of changing the TC....well it may be time for the dealer to make amends.
-Eric