OK, well, your first paragraph is the kind of elaboration I was looking for.
I don't think anyone knows exactly what the issue is. Or why it seemed to only pop up this year and not in previous years.
I find your theory mildly perplexing, in that Bello faced at least 24 batters in each of his "relief" outings, so he would have faced the best hitters at least twice each.
They are basically using Yoshida as a platoon DH. Only 24 of his 196 PAs are against lefties. I don't think much of his game but like every Sox player I root for him when he's in the lineup.
Tracy's in-game management seems fine to me. I haven't seen much howling about it in game threads.
Maybe the one complaint was when he had Bello try starting again and it blew up.