This is pretty much becoming a widespread issue with budget earbuds (even some more expensive ones according to some people). As I've said before, the issue is MX type of design and how much modern membranes are moving resulting in small dents on the membrane when it contacts front cover (at first happens when there is a lot of bass and loud volume). Over time, it builds up until membrane is dislodged and rattling becomes constant.
We pretty much need a radical shift towards a dome shaped front cover - PT15 appears to be exactly that but there is a plastic part below metal which is probably flatter.
Here is a photo of PT15 vs MX front cover (found here, around 1000th page):
Very interesting! Do you have any examples of buds with such a dome shaped front cover?