Thanks guys we'll look into this further!
I just found out a couple of issues with Speed /STrength that I'll ad here in case they're somehow relate to your issue:
I had a hero without weapon in combat, with Amulet of Mighty Fists, and another hero with Strength spell. I cast the Strength first and then it won't allow the first hero to use the Amulet. However, when a canceled, I was perfectly able to use the AMulet first and then the Strength spell.
Also, similar case with Speed and Poog of Zarongel. If I play Speed first, and the Poog (for his +3 Bonus) - Poog negates all check bonuses before that (Speed and the Ranged skill bonus); however, if I play Poog first - I then have no trouble applying Speed on top of it.
I also had one or two cases (before I started to keep track) with those two spells where I was able/not able to cast them off-turm purely based on the order in which I played them and other cards (weapons, bonus item, allies...) - so it leads me to believe there's something wonky specifically with the timing of these two spells.