I was able to replicate the signing bug that prevents unlocking funds from a lockbox on testnet, fresh wallets.

I described the bug in detail on a previous post,
so I'll just sum up here the environment and a path to replicate the bug.

Environment I used included the following ingredients:

• linux mint 19
• Armory, testing branch compiled from source
• Bitcoin 0.16.2, running separately on testnet
• 3 Armory wallets:, "Primary Wallet", "X Wallet", "Y Wallet"
• a 2 of 2 lockbox "TestBox" created using public keys from "X Wallet" and "Y Wallet"

Path to replicate the bug that I've used:

• Fund Primary Wallet with test coins
• Open Lockbox dialog, copy funding address, Segwit checkbox cleared
• Fund the lockbox from the Primary wallet to the above address, wait for confirmations.
• Empty the lockbox back to the Primary Wallet, using X Wallet and Y Wallet to sign. There should be no problems.
• Open Lockbox dialog, Check Segwit checkbox, copy the address.
• Fund the lockbox from the Primary Wallet, to the lockbox segwit address. Wait for confirmations
• Attempt to empty the lockbox back to the Primary Wallet, and the bug will be manifested. It is impossible to unlock funds from the lockbox

This proof strongly suggests that users should not use lockboxes with segwit checked, as Armory will not be able to release funds from the lockbox.