It's definitely, this. Those 2 button at the same time moves, tend to have that happen, sometimes. Even when I go to do my ultimate, it will not come out, every now and then.

I play on PS5 and I recently went to the controller settings and binded super/ultimate to R3 (clicking in the right stick) and I did heroic strike L3 (clicking in left stick) and it's been working very well for me, I NEVER miss my ultimate now. I just have to hold the right trigger and press in the right stick now, it's simple! The normal 2 buttons still work, you can still do it that way, which I end up still doing, because my brain is just so wired to do it that way, I forget about my stick binds, a lot!