This is more of a potential problem that's up and coming, rather than a problem right now (msgpack v1.1.5 tag and mspack v0.5.5 as seen https://github.com/hashicorp/go-msgpack/tags). The upstream msgpack has changed how time was encoded/decoded and the workaround was provided.

I suspect either two options, decode the data with TimeNotBuiltin set and then encode it without, allowing a stable migration or it might be prudent to expose TimeNotBuiltin in the raft configuration (under another name LegacyEncoding or similar), so this can be changed for users with existing data.