I've built a dark room general mob farm above Y=200, floating above an ocean with no islands with an 8 chunk radius (easily). General mobs rarely spawn in the farm... occasionally spawning stops all together.
I've built this farm many times over several version and it has always produce orders of magnitude more output than what I'm seeing. The world is a fresh seed and is not upgraded/converted from a previous release or snapshot.
Side note: I've been able to find numerous related reports for the Bedrock edition, but I can not find one that addresses this issue for the Java edition. I'll bet dollars to diamonds that it exists and a mod will mark this duplicate within the hour. This is basically my best way of "searching" for an open report on this matter.