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I took my Miraphone Firebird tuba in to a local shop to get the valves realigned and the original cork bumpers replaced with Neoprene. The material the repair tech used is SUPER bouncy, to the point that I can't play slurs without the sound being interrupted by the rebound of the valve. It's an orange-ish color. Not sure what it is, but it wasn't what I was expecting. Not to mention, they didn't align the valves properly either, which was the whole point.

What should I do at this point? Obviously I can't keep the valves as-is, but should I go back and ask for a different Neoprene material? Just get cork again? I was hoping to switch to Neoprene for the forseeable future, but this clearly won't do. Has anyone encountered this type of material before, or even just a similar situation?

(I'm a college student, and pretty inexperienced with repair. Just looking for general guidance.)

I'm not entirely sure of the different types of Neoprene, but I've never had Neoprene bumpers that didn't bounce like that. It's not so bad with lighter valves, but if the rotor itself is heavy enough or there's a lot of spring tension on it, it needs something more substantial than Neoprene to soak up all that rotational inertia. All of the bumpers on the rotors I have (which, to be fair, are all on trombones) have rubber bumpers, and those valves (including the oversized rotors on my bass trombone) don't bounce. If Neoprene doesn't end up working out, I'd consider rubber bumpers.

That "Orange stuff" is silicone O-ring material your repair shop got a McMaster-Carr. You are 100% correct about the excess bounce. A lot of shops love that stuff as it is much easier to cut and trim. I keep some for an occasional trombone player request.
If the shop where you had the work done is run by decent folk, go back and show them the excessive bounce. With the heavier springs of tuba levers, it just happens. Ask them nicely to use Buna-N O-ring cord stock (I think 6.5mm) on your tuba. Also available at McMAster-Carr. After 2 years it gets hard but by then your horn needs servicing again anyway.
I bet they don't charge you for redoing it.
I get the bumper material in a half dozen different metric sizes so that I minimize the amount of trimming I do.