Likely bugged statue
Statue with description "On the weathered stone you discern the features of a tall elf, his right arm holding a lute, his left distorted in a mass of tentacles, a sly grin on his scarred face." and inscription "All hail Tarindil, master of music, embracer of ChAoS!" seems to have bugged effects. Or in case it's not bugged, it's extremely powerful.
I did some testing with the statue and got following results:
PC with no corruption got 1 corruption level. (this seems like intended effect)
PC with about 0.5-1 corruptions got some corruption points removed.
PCs with 5 and 17 corruptions were cured from all corruptions (but left with some corruption points).
And maybe the weirdest result, but possibly intended: PC with just above 1 corruption first lost the corruption and immediately (on the same turn) regained a new corruption.
Attached save is next to the statue in extremely corrupted state (17 corruptions).