Verify Mycelium still has private key for spent address
Mycelium uses a hierarchical deterministic (HD) wallet, which means all private and public keys follow from an initial master key value. Every time a spend transaction occurs from the wallet, you can see that another private key is generated from the master key and it keeps a count of how many private keys are used. Any application that can sweep such a wallet should be able to acquire all the private keys along with it. If not, other questions answer how to extract private keys from an HD wallet: Derive private keys from a HD key