I've noticed that when I'm fighting a energy heavy mech and they've drained my energy (not the shield) their weapons do full damage.

Has anyone else experienced this?

Lol...duh when u have 0 energy the energy weapon's drain turns into damage.SO THE DO OVER DAMAGE

Thanks for explaining it, seems a little unfair since when they run out of bullets/rockets it doesn't do that.

Technically U cant shoot anymore....

#heatmechs4lief

I was fighting a mech that didn't use bullets or rockets, so shouldn't I be doing more damage to them since they're "drained" of those per my OP.

I cannot follow your logic

Your weapons didnt drain Bullets or Rockets.
Hehe no weapon can drain Bullets/Rockets though.

I switched my mech's weapons to all energy, and now I fight mostly heat based mechs.

Mostly.

I created a new mech recently and was fighting level 3 mechs with mythic and legendary items. Their HPs two or three times my own.

Ohk i guess i get what u are trying to say , u most likely mean that why doesnt Physical mechs get extra damage when enemy runs out of bullet, the main reason being

Physical or rocket mechs already have enough damage , u can easily find lot of physical and rocket weapons with 120+ damage , but when it comes to energy mech they dont have the powerful weapons , the sword does some 90 damage and the shot guns and other weapons would stand between 60-80 damage and all those weapons drain the users mech energy too hence he is not able to use them all the time , hence the extra damage they get is very important cause that makes the diffrence for them.

Atleast you are lucky that an energy mech would leave u still capable enough to do a stomp , unlike those heat mechs that would leave u standing turn after turn.