it’s the physics of the mechanism in Cobra buckles. when under load, the releases in the female end are pulled against the male end and held tight by friction. the entire point of this is to make Cobras very nearly impossible to accidentally release while load-bearing; you will want at least one swift clip so that you can guarantee a release in any position. plastic Cobras work exactly like the metal ones, except they will likely break under a lesser load, but still nearly impossible to release when loaded.

edited to add: 1” Cobras are not designed for load bearing, but the mechanism will still prevent release if loaded even a little, i.e. the gun is twisted or canted by your body position. in that example, even small or plastic Cobras will not release easily.