Bar line at end of multimeasure rest cannot be edited
When attempting to change the regular bar line between two multimeasure rests to a double bar line, Musescore 2.0.3 will instead separate one bar from the beginning of the first multimeasure rest (shortening the mm-rest), and has a slight chance of applying the double bar line to the end of that bar instead (it does not appear to be deterministic, which is fairly frightening...)
Workaround: Turn off mm-rests before attempting to edit this bar line.
Example: In the attached image, I attempted to edit the barline between bars 27 and 28. Selecting it, then double clicking the double barline in the basic palette, will separate bar 20 from the first mm-rest, and end it with either a single barline or a double barline, at random. Continuing to attempt to change the barline will repeat the process and has a random chance of going from single barlines to double barlines in the newly separated bars.
Seems I remember encountering this but I don't see an existing issue for it in the tracker, nor does it seem fixed in the builds for 3.0. Could you file an official bug report for this (eg, Help / Report a Bug, from within MuseScore, or the "Issue tracker" link in menu at right of this page).
In reply to Seems I remember encountering by Marc Sabatella
Thank you, created: https://musescore.org/en/node/166361
In reply to Thank you, created: by syntaxaire
Or even #166361: Bar line at end of multimeasure rest cannot be edited :-)