I have the same problem, plex is not using my custom certificate and defaulting to the *.plex.direct cert.
I generate the certificate bundle with acme.sh: acme.sh --to-pkcs12 --password ##### -d my.domain.com
From the plex media server log:
Apr 04, 2023 11:43:18.316 [] INFO - Plex Media Server v1.32.0.6865-ba892e5ce - Docker Docker Container x86_64 - build: linux-x86_64 debian - GMT -05:00
...
Apr 04, 2023 11:43:18.360 [] ERROR - [CERT] PKCS12_parse failed: error:0308010C:digital envelope routines::unsupported
Apr 04, 2023 11:43:18.360 [] ERROR - [CERT] Found a user-provided certificate, but couldn't install it.
