I used to collect these and keep them in a coffee tin filled with dirt (and air holes) they bred like crazy. I loved those little rollie pollies so much!
It was the 90s, insects weren't even on the radar of animal cruelty. Parents definitely weren't commonly teaching against it. Before anyone @ me. I disagree with it now.
Either that or it goes into whatever birds or whatever other bioaccumulators that consume them and accumulate it at the top of the food chain (maybe eventually you and me).
There might be some immobilization, but it would be relatively short lived, depending on the C:N ratio of the isopods ( I can't believe I just typed that) and where the metals are stored in the isopods (again). If it's in the chitin, release times would be slower.
Either way, it would ultimately return to the soil. Some organically bound metal (e.g., in soil organic matter) are labile and might leach out over time but the bulk of it would be stable, I imagine.