Hmm. Seems like the problem isn't actually as big as I'd thought. From what I can tell, the forwarding bug was only actually happening in the specific circumstance where the message to be forwarded was coming from the same address that it was supposed to be forwarded to. So, essentially, here's the arrangement:
I mainly operate through account A. I also use accounts B and C for various things, and B and C are both configured to auto-forward messages they receive to A. Using A, if I send a message to B, I get it redirected back in my inbox on A. Using A, if I send a message to B+1, I don't get it redirected back in my inbox on A. This led me to conclude that the issue was with redirecting messages aimed at B+1. However, I just ran a followup test, in which I used C to send a message to B+1, and it redirected to A perfectly well. So apparently the problem happens only from A to B+1 back to A, but not from A to B back to A or from C to B+1 back to A. I'm... not sure what that means about how gmail is programmed, but it does mean that I'll be able to use non-A addresses with plus signs without any issue. So problem solved, I guess.