If the unit is setting on top of 3/8 shims and someone sits on the edge without continuous support under it, yes it can crack. If the shims where providing continuous support then I would guess the unit is too thin and you should notify Kohler to get repair or replacement.
Some of your description confuses me. Such as this is a drop-in unit being used for a shower and you installed it with 3/8 shims on top of the surround.
Is this unit being installed against 3 walls (alcove installation) or does it have a deck around it? The "surround" is IMO considered the walls around the tub, so I assume you ment you installed it on top of 3/8" shims on a deck. I take it the 3/8" shims are for tile so the whirlpool needs to be within 1/16" from the finished deck for the caulk joint. If this is installed in a deck, how are you going to use the shower? A wrap around shower curtain? Anyway, do not put any pressure on the outside edge of the unit until you have the finished surface under it within 1/16". Good you set the unit in a mortar bed for better support.