Owned games still showing in giveaways.

Steamgifts can't properly check for DLC or for games that have been removed from the Steam store.

You can hide games you don't want to see. It's an icon of an eye with a line through it.

Thank Valve's "Steam is learning about this game..." hiding the game from the API, the asset Steamgifts uses for ownership assessement.
Lovely, ain't it?

This can happen for games that are:

1. Removed from steam store
2. DLCs
3. Deluxe Editions / Complete Packs / GOTY etc
4. New (aka 'Steam is learning about this game')

Just hide them manually.

If you use ESGST you can use Hidden Games Manager to add all owned games to the SG filter list with a few simple clicks.

that's quite normal though
even when hiding , it still sometimes showing

Sometimes happens indeed, like others point out due various reasons. Best way is to just manual hide them, by clicking the eye next to a giveaway name.

View attached image.