Never heard of strut mount bulging one way or another.

Typically a knock in a strut following a replacement job is due to the journal (ledge) on the strut piston not fully contacting the bottom of the strut mount.

Solution is to tighten the top nut and/or add a large washer or two under that nut to draw the piston journal tight up under the strut mount.

This is almost essential when using KYB strut inserts, I have found.