After further investigation, it looks to be related to the new DNF module feature in EL8.
Workaround is to disable the postgresql module:
dnf module disable postgresql
After which, postgresql (ol8_appstream), postgresql10 (pgdg10) and postgresql11 (pgdg11) are installable.
Still need to look further into how these modules work and if any packaging changes could be done as a better fix for PGDG.