Leather Cuirass Lorica Musculata, I used to think no way but

[Dan Howard]

Dan, the flak vests were next to useless. The stuff that was proven to work, or we purchased, we used in the field. The ballistic vest was a known lifesaver, and it was light and comfortable to wear for hour after hour.

You've just made my point about flexible leather armor. If a Roman was issued something that didn't actually offer any protection there is little chance that he would wear it so we would not see it in sculptural evidence. Therefore the sculptures cannot be depicting flexible leather armor.
It could be flexible leather triumphal regalia
It could be flexible leather parade gear.
It could be fleible leather halloween costume
It could be flexible leather bondage please whip me ooh more please...
It could not be flexible leather ARMOUR - it didn't exist.

Leather can be made serviceable for the battlefield. It was done so countless times by many cultures in many time periods. Not a single example was ever as flexible as the stuff allegedly depicted in the sculptures. It is physically impossible to get leather armour to look like the literal interpretations of these sculptures.