Does your Glock have the NY1 or NY2 trigger spring installed? If so, I believe this is normal behavior, as the trigger must be depressed again (or held down) to remove the slide on Glocks that have these components installed.
If you have your Glock case with the sticker, describing the gun, it should say "5.5 lb", "8 lb", or something similar on the sticker. The 8 lb spring in known as the NY1 spring, and there is also another, heavier, spring called the NY2.
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