Is it ok for a bathroom sink vent to go up and come back down over an obstacle?
All vent and branch vent pipes shall be so graded and connected as to drain back to the drainage pipe by gravity.


which this arrangement arguably does have.

I believe that as the code is typically interpreted, the latter half is draining &quot;forward&quot; (in the direction of the vent) rather than &quot;back&quot; even though the water (condensation, mostly) ends up in the same drain pipe ultimately.
Practically speaking I can't see that it should not work, and IPC tends to have so many ways to vent things I would not swear it's not allowed in some paragraph I missed (or found not relevant to my situation) when educating myself for my own plumbing, but I don't think it quite passes.