Rapid change occured before, but it went away after reinstalling it, I was afraid this might be a variation of that bug again.
Good to know it’s intentional, I assumed that something must be wrong with the MFD - seeing time to periapsis change a lot while periapsis itself barely changed.
No, it is just the way how the data is displayed - instead of calculating a real and non-kepler orbit, or a filtered orbit with proper elements, it displays the Kepler orbit passing through your current position with your current velocity vector.
That is much more CPU friendly, but of course, requires some understanding of the data displayed (you essentially are required to do the filtering in your brain, what works pretty good usually with some experience) and to use a more careful approach to maneuvering.
Instead of a single hail mary maneuver, you carefully approach the target - and the closer you get, you and the target will get more similar osculating elements, with the differences to the real orbits becoming similar as well (Since you and the target are subject to the same perturbations, if you would be in the same position at the same time, after all. And you will get almost the same perturbations, if you would just be at the same position as the target at a slightly different time, like 30 minutes later)
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What you might want to disable is "Nonspherical gravity sources" on the Parameters tab.
Does not change much, since sun and moon are also producing differences between an osculating kepler orbit and the real orbit.
The behaviour will just get much weaker close to Earth.