I followed all the steps to make my mob grinder and the tutorial was in 1.15 (Mine was too), he said it can be done in all versions of Minecraft, I even double-checked it. I'm sure I had lit all of the caves in a 148 block radius. The inside of the grinder was completely dark. The roof was very well lit. No matter what I had done, it still didn't work. Here's the tutorial:
You probably did nothing wrong. There is a known bug/behavior since May 2017 that mobs do not despawn as they should in bedrock edition (https://bugs.mojang.com/browse/MCPE-21856).
Today (17 March 2020) it was reported that mob farms are working. On that note, it would be really helpful if someone could link to a savefile with mob farms built so more people (myself included) could readily test and confirm that this issue is finally resolved without having to build a test world from scratch.