Hello,
Welcome here, HippieNaut !
Me too. But whatever the spacecraft you use... and the Orbiter version.
For exemple, Same thing happen in Orbiter 2006, 2010 and 2016 with the single scenario "DG Mk4 in orbit" (with the default DG). Making a burn in orbit normal +/- make the ApA (and PeA a little) rising.
With or without graphic client and/or flight perturbations in Parameters tab.
Windows 10 ?
Milouse