User @7h31ll3g4l posted about the issue on reddit and numerous others have corroborated the report.
@IrishSmurff - Same here, updated this morning, iPhone 13 on 15.3, watch series 6 on 8.4, passes on phone not syncing to watch, but deleting pass on watch wipes it from both
@magebarf - iPhone 12 and Apple Watch Series 3. No coupons/membership cards added on the iPhone have successfully been synced to the Apple Watch since I updated to 8.4 and 15.3. Interestingly enough, removing a card from the iPhone does not seem to sync to the Watch either, but removing a card that exists on both devices on the Watch successfully syncs to the iPhone.
There are also multiple threads on Apple's discussion forums about the issue.  
Until Apple resolves the problem, share a pass through iMessage to yourself. Then on your Apple Watch, open Messages and add the pass to Wallet.
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