So it took me a minute to realize you were specifically talking about applying a vSAN license PAC to ESXi hosts.  Got it.  

I have only encountered that issue when upgrading from trial to full licenses in a lab environment. 

vSAN licenses are per core, and definitely vary per version lately. But a PAC shouldn't run into those issues.  It's possible that the vendor used the wrong PAC, or more likely an outdated set of codes.  VMware has gotten tighter about license keys ever since they killed off the 'engineering' licenses that used to be good for 90 days at a stretch.  That's where I ran into things, when that engineering (trial) license expired we had to work with the vendor to secure the correct PAC and get our lab gear fully licensed (because we were promoting a prototype into production as a solution).  

It sounds like you're stuck waiting on your OEM/vendor to resolve the issue, but if your purchase included any VMware 'pro' credits (if they still do that) it might be worth activating that to get a VMware engineer on your side to help address the issue outside of the normal ticket process.