Thank you for your reply.
Yes, the rivet next to the button is the LP4-3. I put it in the picture just for dimension reference.
The AB4-6 is MUCH longer. It is about 13mm long (0.525")
I do have two more of them.
The rivet's grip is 8-9.5mm. The thickness of the joint parts is about 8.5mm
Here is another photo from a different angle:
It looks like the shop head has normal size. So it cant be an LP4-X rivet installed by mistake. Longest LP4 rivet is LP4-5 and it is quite a bit shorter than AB4-6.
It seems that the size of the rivet and its grip is correct for the application.
That and the fact that all six of them behaved the same way almost looks like a normal behaviour of this particular rivet.
I wonder if anyone else observed the same effect after having the SB completed.