It _was_ caused by an old version of RealPlumes that had an incorrect effects definition. It should be fixed in the recent versions of RealPlume.
But any mod could cause it that doesn't setup their float-curve configs properly.
My re-entry profile for Mk2 spaceplanes is this:
1.) Get into LKO. Serious. Dont' try re-entering directly from Mun, Minmus, and certainly not inter-planetary. You want orbital velocity <2.5kms
2.) Retro-burn to place your PE at ~45km. You want to bleed off most of your speed -above- this altitude.
3.) Orient prograde with the highest angle-of attack that your plane can do.
4.) Bleed of speed in the upper atmosphere. Let the aerobraking drag your AP into the atmosphere. You might even see your AP increase a few times depending on your AoA (skipping off the atmosphere). PE will continue to decrease due to drag and loss of speed/energy.
5.) Patience. It might take 3/4 of an orbit or more to re-enter (depending on drag profile). This is -not- a precision method. Eventually you will be going slow enough that you'll just kind of fall down into the thicker atmosphere where you can start flying like a plane again.