I've experienced the same in Rustlers and I believe the issue is chassis flex. Even with the pack held tightly they packs end up curved or the solder leads break off. It's just the nature of the plastic chassis.
The old Rusty's used to have an optional aluminum chassis plate that solved this issue. Stick packs also seem to fare better than side x side configurations in Traxxas trucks.
GV2 Pro, XXX4G+, Blitz SCT, CRC GenXL