I guess I'm more thinking about Ruby/Rails conventions, e.g. methods like Hash#symbolize_keys.

Mixing the two has always been a smell to me, but maybe you're right.

Sure, but JSON.parse returns a Hash, which is key-value pairs. The method argument is about the return value, not the input value, so it is more like "Parse this JSON, and symbolize the keys in the resulting Hash before returning it to me". Plus, as mentioned, symbolize_keys is more conventional.

The issue was triggered by using multiple parsers, with different behaviors.

The change of behavior here would prevent the specific problem, but makes the general problem worse.

This change creates a new code path that did not exist when all the app code was developed and tested, so there's a decent chance something bad can happen that it's not prepared for.