It could possibly be a bug within the app, however let’s think about what you might be telling the app to do.
For those who added a passphrase to a single WIF personal key by way of a BIP38 passphrase (merely encrypting the personal key) you continue to have the identical ensuing public personal key pair upon decryption. So you might be telling the app to create a multisig pockets with the identical public key (which actually is not multisig anymore). Additionally somebody solely must know one of many passphrases to spend since all of them resolve to the identical public personal key pair.
There may be nothing technically flawed with this, you completely can add the identical public key 3 occasions, and require 2 to spend. I do not know why you’d, however it’s “allowed”.
I imagine the outcome you might be in search of could be extra simply achieved with a BIP39 mnemonic and including a separate passphrase to every. Completely different passphrases on the identical bip39 mnemonic will produce a distinct public personal key pair for every passphrase. This fashion you really need the correct quantity of passphrases to spend as an alternative of only one.