My 5906 is out in the garage so I pulled my 5904 out of the safe to see exactly what you're asking about. Low and behold, the front of the slide does move downward towards the frame when you apply pressure as you describe. It's not much though, maybe .010" or so. Keep in mind, the 5904 has an aluminum frame. I'd say that there are no worries, it shoots just fine.
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