Was the grass from a treated lawn? Sounds like perhaps a bit of herbicide damage from the grass cuttings. One has to be careful about what was put on the lawn if one wants to use the grass cuttings for mulch. (It is even possible that some of the clippings may have carried wind drifted treatments from a neighbor's lawn treatment even if one's own lawn was not treated!)

My 'lawn' wasn't treated by evidence of the seeding dandelions I had to weave thru with my mower when gathering the mulch. It is more a pasture/garden than a lawn but I keep it cut. It is possible that my neighbor may have sprayed but I don't know. There were Broccoli, Garlic, and other onion/shallots nearby that were not affected and I did mulch one of the broccoli beds with grass from the same cutting but I left more space between the mulch and the plant stems. I even fertilized the broccoli with urea before mulching them.

Herbicide presence and drift is something I have to worry about here in the City; I have had spray drift aroma noticeable from two and more lots over. While I was growing up on the farm we used a weedspray named Amino Triazole (3-AT) that turned treated plants white as it killed them. I sprayed more Canada thistles with that in both row crop and pasture than I dare to remember. That is what your problem reminded me of. 3-AT has been mostly replaced by RoundUp, which is more effective and said to be safer, although probably not actually safe either, if you really get down to basics.