|Great images and info, thanks. Note the emphasis I added to your post.
The problem is that the new fastener has a 'spacer' on it that makes it so that the bolt doesn't seat in far enough into that counter-bore that you call out. Therefore, the nice, slick nickel plating is in direct contact with the tripod receiver and allows the plate to slide even after the lock lever(s) are snapped into place.
If you take a measurement of the bolt head on the original fastener of the 488RC2 plate there, you'll find that it's .196in (5mm) high. When the bolt is seated into that counter-bore, the distance from the surface of the plate to the top of the bolt head is .157in (4mm). This leaves, roughly, .028in (.711mm) of clearance between the bolt head and the tripod receiver if the overall depth of main the cutout is .182in (4.66mm)
On the T1 fastener, the bolt head is ALSO .196in (5mm) high, but there is an additional 'spacer' on top of that [seen in the op image] that is another .039in (1mm) high and it's this spacer that slips into the counter-bore. This leaves the entire .196in (5mm) height of the bolt head sticking above the plane of the qr plate's surface.
If you then factor in that the main cutout under that plate is .182in (4.6mm), you have .014in (.355mm) of the T1 fastener extending past the cutout and giving "slip" to the whole unit when it's mounted. .014in (.355mm) is a very minute amount, but when compressing a nickel plated surface down onto another, hard surface like this it's enough to cause slippage.