Yes, this indeed is a bug.
We were moving a forum to another server last week and we ran into the LetsEncrypt reissuance limit (max 5 per week for the same hostname). At first we had no idea why but this bug caused the certificate to be reissued on every build and after five times it ran into the rate limit. That didn’t ring any alarms because the previous certificate was still on the server.
Until we moved the forum to a new server. Then we did not get a fresh certificate. We could copy it from the old server but we never realized what caused it.
acme.sh is pinned to 2.9.0 whilst master is at 3.0.1 and has a feature for setting a default chain, which I suspect might be related.