I know for a fact that my skimmer isn’t causing this issue because I’ve left it off for a week at the problem continued as if nothing happened.I had a similar problem in my DIY sump where skimmer microbubbles in water flowing over the low wall of the low/high bubble trap made it under the high wall. I solve the problem by adding a piece of porous foam at the opening at the bottom of the high wall bubble trap where the water enters the return chamber. If you have a powerhead in the return chamber, it will likely keep the microbubbles suspended long enough to get sucked into the return.
Nevertheless. I’m closing my skimmer for another week and I’m closing the pump that was in my sump. Let’s see if anything improves.