That math works. 30ms per channel sample. 14 channels (12CT,2VT) = 420ms between channel 1 samples and 30 ms between chan 0. So two missed chan 1 samples. The difference in voltage drop is about 14:1. It’s altogether possible that the problem is much more frequent than you are reporting and that it only shows up significantly when it happens during the relatively infrequent channel 1 sample. 30ms of zero on channel zero is a blip.
Also, it’s different code to sample just a voltage channel vs a CT/VT pair. It’s possible there is something there.