Sadly after much frustration and confusion I have found the answer to my own question.
The .pkpasses file type is only supported by iOS 15 and above.
The documentation linked above makes no mention of this at the time of posting, but after upgrading my test iPhone to the beta version of iOS 15 I am able to successfully add my .pkpasses file to Wallet.
Hopefully this saves someone else from going through the difficulty of finding an answer to this question.
jamespryde_ts
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