Voice is dropped in ads on cable channel via OTT
I don't have a definitive answer, but what you're describing sounds like phase inversion on one of the stereo channels. This causes cancellation when heard in mono of anything centered in the mix, as an announcer voice would most likely be. Someone listening in stereo would not hear much if any difference, so I assume you're listening to a mono feed. It's hard to imagine a scenario where a commercial is delivered in a way that would separate the main channels but leave the background sound.

Why only sometimes? Speculating again, there may be one misconfigured server in a group of servers, and playout of any given spot is randomly coming from that server. It's also likely that these are local insertions, since an issue like this would probably be detected quickly at a national level.