Games I already know showing up for giveaways

Hide them.

You know what would be helpful? Telling us which game/s seem to be the problem!

did you bought the game recently? try sync steamgifts with steam :)

Forgot to sync or maybe profile limited gamed?

either you need to sync or its a package(deluxe edition/etc..) and the steam API doesn't report those so it won't show as owned.

added some info in my post guys! But here it goes:
It happens with ALL games, not specific ones. I have +- 2k games in my library, so it is a no go to block every single one.
My profile is not locked, all is visible.

Check here https://www.steamgifts.com/account/settings/giveaways to see if you need to change #2 to Yes, perhaps
Edit: I see someone was faster than me, I must have been on this page longer than I thought lol

This is bit crapshoot but; Try putting your playtimes public, force sync over here, see if stuff changes. You can change playtimes private after again.

It shouldnt work, but lately steam started to hide free games with no playtime on api. It shouldnt affect "paid games" but you never know