I bet it's River. I haven't heard of anyone else re-using keys, but I heard in an interview with the guy from there that they ONLY use bitcoin addresses that they know they can send bitcoin from (re-using). This transaction left some BTC behind and the source that populated it also has funds leftover. It seems like that kind of behavior since River's whole model is that you buy bitcoin and they send it to your own wallet. So that would track with all of these tiny transactions going out of it--people buying $50 worth of BTC and getting it sent to self-custody.