Solid infill artifacts - what was happening here?

Any idea what was happening there? I was pretty sure it’s a belt problem (and it partly was and maybe still is in parts), finally decided to just change the belts, which was sorely needed but didn’t fix the issue. (PSA: Change your belts sooner rather than later, when they harden due to heat and IPA exposure in the toolhead they are a major PITA to get out!).

What fixed the issue was swapping the extruder+nozzle (not sure which one of them it was, my bet is on extruder, I’m pretty sure I tried 2 other nozzles!).
The weird thing is, if you let something just print, you wouldn’t even know there was a problem (unless it was a cube). Even the walls were pretty nice. But solid infil surface looked like this.

Any idea what could have caused this? It’s not underextrusion per-se, but instead of making nice flat linec, it made something taller which interfered.

Oh and I only noticed it on Bambu Matte PLA - some filaments simply didn’t do it, like PETG which printed just fine.
I’m at a loss to explain anything…
Bottom: before
Top: after
Me: gobsmacked

I might have congratulated myself too soon - another print, same artifacts. No idea.

If the extruder steps/mm were not properly calibrated, it could result in inconsistent extrusion, leading to uneven infill surfaces. Recalibrating the extruder steps/mm might help achieve more consistent extrusion. If the nozzle diameter was not consistent or if it was worn out, it could lead to irregular extrusion patterns. Swapping the nozzle might have provided a fresh, more consistent extrusion path, resolving the issue.