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.