Do PBKDF2 keys generated at a lower iteration count leak information?
Knowledge of server_password computed with 3,000 iterations instead of 50,000 makes it over 16 times faster to brute-force inputted_password and find encryption_key than was otherwise possible.

server_password can take much more values ($2^{256}$) than inputted_password can be tested, and PBKDF2 behaves like a random function, thus the first inputted_password found matching (if any) is the true one with overwhelming certainty.

Having found inputted_password, the adversary computes encryption_key by applying the same relatively low cost technique that generated it in the first place. That also allows to check that inputted_password is right (that is, the user did not misskey while s/he was tricked to use a lower iteration count; but even if that was, that could be worked around by trying small variations).