Syndra and Taliyah didn't cast a second time even with full mana (video)

I've seen this happen to me before, but managed to get a video clip of the round it happened on this time.

Maybe I'm missing something, but both of them have full mana (from what I can tell) and continue to auto attack 1-2 more times without even bothering to cast. There's no hush or anything that should be stopping them from casting so I'm not sure what the deal is. Maybe something to do with Zyra plants and targetting? I'm not really sure, but unless I'm blind and missing something this clearly doesn't seem like it's intended. I will say, it seems Taliyah may have cast before but her mana never drops for some reason, so that's possibly a visual bug, but the Syndra never casts again for some reason.

Actually, while typing this I went and found another round against the same player. Turns out I died to them and the same thing happens with Taliyah (Syndra got wrecked so she didn't have a chance to prove it again), it's a little harder to tell though because she dies pretty quickly after she gets full mana. But, it looks like she gets full mana and goes for another auto attack instead of casting again, but dies shortly after so I'm not sure it's as clear as the video I've linked.

Checked some other rounds and they seem to be casting fine, so doesn't seem to be an issue outside of the ones against the summoners.

I'm happy to share the other video if it can help! And would also be more than happy to hear an explanation that makes this make sense for me. Thanks!