I’ve run into something similar when trying to juggle multiple replay stingers in vMix.
From what I found, SetStingerGTInput1 can be a bit misleading depending on the version—sometimes it won’t actually swap what you expect unless the Stinger is already properly assigned to the overlay channel and the GT title parameters are correctly exposed. In a lot of setups, people end up having better luck controlling multiple stingers by pre-loading them into separate inputs and then triggering them via shortcuts rather than trying to dynamically swap a single stinger slot.
Also worth double-checking that your titles are actually exposed to GT so vMix can “see” them as switchable elements.
I came across a breakdown that might help clarify how stinger assignment and triggering behaves in practice on this website (worth a look for how people are structuring layered replay graphics workflows).