PoE causing alt key to become permanently depressed

Since Sunday, every time I launch PoE, within 10 minutes of loading up the game, my alt key becomes permanently depressed, persisting even after I exit the game and only being rectified by a computer restart.

I have cleaned out my keyboard (it was due for a cleaning anyway) and this only happens while playing PoE.

I have unbound my alt key from doing anything in PoE and it is still happening.