Bug Report: Gitaxian Anatomist

[Muldskogen]

When Gitaxian Anatomist enters the battlefield, you may tap it. If you do, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

We rolled back the turn 3 times and it will not let us tap to proliferate upon entering the battlefield. It shows the ability on the bottom left, but I cannot choose it nor can I choose the card to tap it at any phase from beginning of turn to end.

Thanks for Xmage. It is great. I hope this can be fixed!