Don’t have a solid answer per se - but when I grew autos, I noticed this was quite common among several of the auto strains I tried. I ended up just giving up on trying to find a culprit. Ive never seen it on any photo periods I’ve grown since then. I chalked it up to being an autoflower thing. Seemed to happen around the 2-3rd week of flower if it did. I think I even had a grow question on it way back. I wouldn’t stress it too much if it’s the only symptom happening, and you’re confident your variables are on point