Layer Separation Where A Previous Layer Ends I have a problem with my prints having a small separation gap between layers when I have a corner and the Z layer ends or starts for a partial distance. The gap reminds me of a gap between a print and its raft. Usually 1 layer just seems to be missing in places. 

I've been printing about a year now and I've tried different temps, line width, layer thicknesses. The problem has even been a problem after installing new firmware. I use Cura V4.5.0 and an Ender 3 printer. The problem is rather difficult to describe so I've attached an image demonstrating the problem.
Almost certainly the material that was supposed to be deposited there was lost (oozed out) somewhere else during travel moves due to no retraction, insufficient retraction, or retraction being skipped due to combing. Make sure retraction is on, at least 6 mm, and try disabling combing or setting the max combing distance very low, around 2 mm or less.

If your new firmware is Marlin 2.0, you can try enabling linear advance too. If tuned correctly, it will largely correct for this along with improving lots of other things.