One workout missing from phone
Solution 1:
It appears that as of today (Oct 23, 2020), this is a known bug. Apple is aware of the issue but no GM fix has been issues yet. It's been reported that un-pairing/re-pairing the AW doesn't resolve the issue.
There's a Watch and iPhone OS versions in beta right now, you can try installing one of those to see if it fixes the problem (installing beta OS's has its own risks so be sure you're aware of them before installing).
Otherwise, you'll have to wait until Apple puts the fix out for everyone.