Date: 20160317 (first incident, others have followed)
Deck: Mind Stealer
Card: Old Man of the Sea

Both myself and the AI were not able to choose to uptap Old Man of the Sea during the untap step. Once tapped, whether by ability or attacking, it remained tapped.

I saw one exception to the above description. This was when OMotS was used to steal a creature, then that creature died. OMofS seemed to untapped automatically (AI possessed, could have been by choice, too fast for me to tell).