When you say "jumped into cruise"... what does it feel like the car is actually doing? Any of the following:
When cruise is activated without AP the car continues smoothly at speed. A dash indicator lights saying cruise is on. If you didn't happen to notice the dash indicator you would only notice cruise being on when lifting your foot and not getting any regen... car continues to glide at current speed. So if you or the car is turning cruise on this way, it would not be felt as a jump.
If you've used cruise and it's still on (dash indicator on) but you've hit the brakes so speed is not being maintained, using the cruise resume function would re-accelerate the car up to the cruise speed setpoint (which is also indicated on the dash). This would feel like a jump, because you'd be slowing or driving around (after having touched the brake pedal) ... but then the car starts picking up speed again to the setpoint... is this what is happening in your case?
During the trip when you've noticed this happening have you intentionally used cruise at any time in that same trip? i.e. the cruise indicator could still be on.
For now, suggest that you manually turn off cruise when you're done with it every time as a safeguard.
If you can demonstrate just once a car that turns its own cruise control on, lighting up the dash indicator by itself... pull over, park the car and call Tesla to come and get it - you're covered under roadside warranty.
Might be worth getting a dash cam and stick it where the dash and your hands and all controls are in view. And just run it until this happens. Because if you ask Tesla to just read the car's logs, a shorted control input looks no different than you activating the control.