When you unload the plates and re-assemble them. I imagine this is a known issue since it’s so obvious. Oh, and they also lose their (fits) tag if they have one. Quite exploitable in a couple of ways.
This has a dark side too. If you disassemble a scratched MBR vest, you might lose the entire vest and just get the plates.
The one I noticed it on was “broken” so I guess I got lucky there. My fab skill is pretty high so I think that helped.