I saw
a video recently which I think may explain the actual cause of this bug. In the video it's explained that this happens specifically when the dropped item limit has been met.
This would seem to agree with my personal experience, which is that this happens semi-frequently to me, seemingly at random times but often during invasion events (I usually don't gather up items until the events are over). But I couldn't figure out how to reproduce it, it only seemed to happen while I was playing casually. It would make a lot of sense to me if there were simply too many items in my world whenever it happens.
I was able to replicate this on a journey mode world, although I can't seem to get the steps just right to produce a whole pool of hearts. I was able to get it to spawn extra hearts twice, up to 5 around 3 heart statues, with all the hearts sitting right on top of the statues. I don't have time for further testing tonight, but I'm fairly confident this is the real answer.
I did get a whole pool of hearts while I was playing casually tonight, it happened during the martian madness event. I was on a broom farming for saucers, and as a result I was leaving a lot of dropped items on the ground. And then at one point I zipped by my heart statues, and there was a whole pool of hearts waiting. (For the record, on the same world as I sent above)