Anet A8 Sudden print quality issue

[BrandonW]

Hello all,

I'm hoping someone can think of something I have overlooked. I have spent a lot of time dialing this printer in and was really happy with it until I suddenly started having problems with layers missing their outer wall. Strangely it seems to mostly to affect recessed walls. The cube picture shows a cube I printed a few weeks ago and the 2 on the right are cubes I just printed using same settings but have annoying flaws. The other picture is the print in which I started having the issue. Hopefully someone here can help cause I'm at a loss.

Things I've tried:

Bed/z axis leveling
Replaced nozzle
Used different filament (different manufacturer)
Check extruder for debris
Tried different spool holder (3 different types)
Calibrated extruder steps
Made sure motors are secure.

Printer Info:
Anet A8 (old acrylic frame)
Glass bed
GT2 rubber/fiber glass reinforced belt
Polymer shaft bearings