How can I fix this bug in the game's orbit physics?

I recommend you lower the SOI size if the ship enters a fixed state, since SOI's should not touch the parent body

Hi, I had a similar problem a while ago, but maybe the solution is the same. In my case, the same problems you explained happened when I tried to leave the SOI of an custom planet. Or with other words, when leaving the planets SOI, speed dropped down to somewhere near 150m/s and won't change.

To solve this, I had to change (in my case lower) the planets SOI in PlanetStudio. So my Tipp is, go to PlanetStudio, load your Planet System, highlight your problematic Body, and there is an slider to change the planets SOI in percent. Try to lower it to 50% one or two times (it should snapp back to 100% after every change) and try.

Hopefully it helps or gives someone with more experience the hint to solve this.
So long

@Zenithspeed well, thanks for your help, for now I'll wait for another game update in case it gets fixed.

@LurinEsparrago oof
if the error persists even with the solution i've figured out myself, then unfortunately i can't help any further since i haven't encountered the bug in months

@Zenithspeed I have done what you told me, but the error is still there, I have also noticed that when I start the engines, the ship stays at a fixed speed, from there it does not rise any more, as if it were inside some liquid.

i've previously experienced a very similar issue that i named the "engine cutoff bug" (ECUB), it also involves entering planet SOI's that causes the orbit to break and makes engines not work (i've made a few posts about it, will add links soon)
it's quite possible that we're dealing with the same bug, but strangely, for me the bug has ceased to exist since around version 1.0 came out and i can enter all planet SOI's fine without the ECUB happening, but strangely it still affects you somehow

i do know a temporary fix for this though:
you need to make a simple planet (blank template works best) and add a launch location on it, and i suggest you also set its size to the minimum
then, add this planet as a moon to the planet that causes the bug, update launch spots, and save the system
after that, spawn in the temporary new planet and land on the bugged planet and save a launch location, then go back to get rid of the temporary planet and its launch spot from the system editor
you could also try simply adding a launch spot to the bugged planet itself, not sure if that works though

link 1
link 2
link to my solution