"but when I look back at everything in previous years he is marked as single."

Single for tax purposes, but Family for coverage purposes.

This is an issue of jurisdiction. The employer follows a State rule that Domestic partners are allowed/required to be offered health plan coverage and the health plan provider offers it as pseudo-family. That isn't Spousal.

Meanwhile, the tax filing is considered non-spouse, as well. So, the person is Single, but the health plan is Family, and that means two individual HSA accounts. And, as I pointed out, the accounts cannot be used for the other party, as you would do in a true family relationship.

And make sure to read the link references, and the IRS, if the other party was claimed under dependent, because that eliminates the other HSA.