Hello everyone first post here, I'm pretty desperate for help. I made a basic orbit rocket with ~3680dV, plenty to orbit. Shared the craft with some friends, and was suprised to see they had way more leftover dV than I did. At first stage seperation, they had almost 150 more m/s of speed than I did. We tested further by sending the rockets STRAIGHT UP WITH NO INPUT and once again my rocket fell ~100 m/s shorter speed wise. I have absolutely no clue why this may be. I have tried looking for help on the reddit, the discord, but no luck. No one seems to understand what is happening. Ive reinstalled KSP, launched with no mods, deleted Physics.cfg, all that. Still no luck.
Here is a video of speed loss in ascent profile: https://www.youtube.com/watch?v=AnYEx61OOMI
Here is a video of speed loss going STRAIGHT UP: soon