Little evidence that protests spread coronavirus in US - AP News
Uh... Economists, that's unexpected.
So, anyway, the research has not been published in a peer-reviewed journal yet, and the news article actually lists a ton of limitations and issues with this theory. I think it's definitely true that the data supports the idea that in many major cities there was not as large of a spike as was expected, and cases started increasing a bit too soon to blame it on just (or primarily) the protestors. The news article also does a good job of pointing out lurking variables, like when states were reopening, that younger people are less like to get serious symptoms, that younger people are less likely to get tested, etc.
But it does seem like the rise is best linked to a combination of reopening and the protests, not one or the other as some people seem to insist upon.