I have not crafted my 1 Glimmer Alicorn yet, and just finished my last-allowed 4th Unicorn. Glimmer Alicorn still shows in my MRC with "1 time only" as its qualifier, while the Unicorns are greyed out and say "Inventory is full" like you mentioned other maxed out should.
Seems normal, but I wonder if since it's a one-time-only craft, not listed as an inventory limit like the rest (which could/
may change), they've set it to disappear like when we finish a Quest craftable?
We can only ever combine those items once, then Poof! Gone?
If yours used to be greyed out like the rest, though, then hopefully s8 can shed some light on it.