This is likely a known bug, or perhaps its intended, with the non-experimental also, but I've tested it on experimental, and I'm not sure how to find known-issues.
Stacking items with different quality levels will randomly change the quality type of the entire stack.
Take a stack of worn rags and a stack of damaged rags. Split the stack so you have several single rags of each type. Combine them, and note that the quality type changes.
It seems like when you have a larger stack of items and combine it with a lesser quantity stack of lesser quality, it will more often result in a stack of higher quality items.