I have a dozen of those 8x10 holders, purchased brand new 22 years ago at Calumet, selected from a much larger number in the store after depth checking all of them. Every one has always bulged like that when not held tight by a camera's spring back. It's perfectly normal.
What you do need to do is make certain your thumb applies pressure to the ground glass frame at the holder's flap end when removing/inserting dark slides. If you don't, that's where light leaks will occur, not at the slide insertion end.
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