Layer Separation Where A Previous Layer Ends
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.