Doesn't your HRA vanish after you leave your job?

My excess HRA (about $3) was sent to me via check, and now I'm wondering if I didn't have to "use up" my HRA via medical websites (fsastore.com) in order to prevent it from getting wasted as I quit my job recently.

I had like $300 left, and while I did use it up wisely to stock up a bunch of eyedrops, contact fluid, sunscreen, and other supplies, I certainly would have preferred it in cash!

I even called United Healthcare (my old insurance) and they confirmed any leftover HRA funds would be kept by my employer.

If I hadn't used it up, would they have sent me a full check in that amount? I'm really confused.