Use metal studs to frame for drywall around brick fireplace
Trying to use a metal stud in this way is not suitable. First off a metal stud is not used flat like that. Normally the metal studs are used on edge such that the drywall would screw to the narrow face of the stud. Then ends of the metal stud are meant to have each narrow face screwed to a U-shaped channel.

A metal stud used flat has next to no strength as compared to using them properly. Keep in mind that metal studs are just thin folded sheet metal, not some strong support girder.

If you must use it in the manner shown in your picture, even though not recommended, you should cut a piece of wood to fit inside the end of the metal stud so that screws do not bend the thing like you have experienced.