Let me use an example from my own field. The example is also well-known among philosophers, so it has the advantage of not seeming obscure.

Gödel proved that the set of mathematical proofs is a proper subset of the set of mathematical truths. In other words, Gödel proved that mathematical truth cannot be identified with axiomatic provability.

This forces a kind of mathematical Platonism, which does establish objective/ absolute truth. To quote Gödel:

"Finally it should be noted that the heuristic principle of my construction of undecidable number-theoretic propositions in the formal system of mathematics is the highly transfinite concept of 'objective mathematical truth,' as opposed to that of 'demonstrability,' with which it was generally confused before my own and Tarksi's work. Again, the use of this transfinite concept eventually leads to finitarily provable results, for example, the general theorems about the existence of undecidable propositions in consistent formal systems."

This is an example of the assumption of absolute truth -- what Gödel described as his heuristic principle -- leading to a profound discovery. It is very likely that the incompleteness results would not have been possible without this heuristic.