I finally took a closer look at the reverted PR, and it includes a lot of unrelated things. From comments in the referenced zFCP/SCSI multipath issue, adding the multipath field to partitions seems to have been the problem. From my testing with the attached user-data, only one line from the PR is required to support mount options. Either a PR with just the one options line or a PR without the multipath line. The former is less likely to cause problems elsewhere. The latter might address a bunch of other things that subiquity autoinstalls are supposed to support but don't yet.
I finally took a closer look at the reverted PR, and it includes a lot of unrelated things. From comments in the referenced zFCP/SCSI multipath issue, adding the multipath field to partitions seems to have been the problem. From my testing with the attached user-data, only one line from the PR is required to support mount options. Either a PR with just the one options line or a PR without the multipath line. The former is less likely to cause problems elsewhere. The latter might address a bunch of other things that subiquity autoinstalls are supposed to support but don't yet.