1.0.0 - Win 10 - Purple Tether casts inconsistently when stacked

I’m trying to run two creatures with Purple Tether on a team and their casting behavior is not working as expected. Sorry that this is very vague, but the outcome seems inconsistent: sometimes these creatures are copying the spells of adjacent creatures, sometimes they aren’t. I have tried re-ordering my creatures a number of ways, and the only pattern I can pin down is that I have never seen both creatures with Purple Tether cast on the same turn. It’s either one, the other, or neither.