Multisig transaction validation issue between Sparrow and Coldcard Q depending on OS

Hi,

I'm running into a problem while testing multisig transfers on Testnet4 using a Coldcard Q (CCQ) and Sparrow Wallet.

CCQ version : 1.4.0

Sparrow version : 2.4.2

Setup:

• 2-of-3 multisig wallet (also tested with 2-of-5)
• All 3 keys derived from the same seed on the CCQ (separate HD wallets)
• Network: Testnet4
• Signing flow: SD card (PSBT → CCQ → signed file → Sparrow)

Steps:

1. I sign the part.psbt with the first wallet on the CCQ → Sparrow recognises and validates the signature with no issue.
2. I switch to a second wallet on the CCQ and sign the part.psbt again → the CCQ saves a .txn file to the SD card.
3. I load that .txn into Sparrow → the transaction displays correctly, but the second signature is not recognised and broadcasting is not possible.

What's strange:

• This issue occurs on Windows and Linux Debian 13.
• On macOS, the exact same operation with the same wallet and the same transaction works perfectly: Sparrow validates both signatures and allows broadcasting.

Question: The problem therefore appears to be related to Sparrow's behaviour depending on the OS, rather than anything to do with the CCQ or the wallet itself. Has anyone encountered this before? Is there a known difference in how Sparrow handles PSBT/TXID files between Windows/Linux and macOS?

I'm not comfortable setting up a multisig wallet for mainnet use until I understand what's causing this.

Thanks in advance.

submitted by /u/arthuro1er
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