Giveaway order isn't stable

Yeah. I've also noticed this. I think it only happens when multiple GAs are ending at the same time.

I can't help you with that. But I have a question: Why do you see so much informations about the games, like tags and so on?

It has happened as long as I've been here (almost half and four years).
It happens in other places, e.g. the Giveaways Created page.

There's websites I use that duplicate elements when this happens, so I don't mind this.

That's what happens when you make a SQL query with only 1 sort order... Should be ordered by <first criteria> then one more, like <id> (because you can have multiple giveaways of the same game with the same name).
SELECT * FROM <table> ORDER BY <your sort order>,<id>

how you can see the genre of games

I've noticed this in the past, my workaround was to add a one minute offset when creating multiple giveaways ending on the same date ;)

10:00
10:01
10:02
etc.