The Comments-URI validation condition was reversed when the field was made optional.
$found{'Comments-URI'} is incremented only after the current header line has been processed. As a result, the existing condition skipped validation for the first entry and applied it to continuation entries instead.
This allowed a nonstandard first Comments-URI to pass, while rejecting a valid external discussion URI following the standard Comments wiki URI.
Restore the original condition so that only the first entry must match:
https://github.com/bitcoin/bips/wiki/Comments:BIP-NNNNAdditional continuation entries remain unrestricted.