Hi Anh,
While GA events in Analytics don't have seat location information (like Seat ID), the Total Seat Count should still be correct. So if you replace your unique count of Seat ID with total Seat Count, does that address the issue?
If not, there is also an Analytics keyword called Suppress Unsold which, when applied, does not bring unsold seats or unsold counts into Analytics. The use of this keyword it typically reserved for very large GA houses where capacity isn't tracked, but confirm that's not a factor here.
Best,Chris