If one of the stages in your multi-stage design is inactive, and if you then move the order of any of the stages, then the activeness of the stages is not changed accordingly.

E.g. in a 'Three-stage rocket', take the following steps:

1. Rename the last stage to "Booster stage 2" just for clarity
2. Disable "Booster stage 2"
3. Move "Booster stage 2" in the component tree so that it's after "Sustainer" and before "Booster stage"
4. Instead of "Booster stage 2" still being inactive, and "Booster stage" being active, the roles are now reversed. "Booster stage 2" suddenly becomes active and "Booster stage" inactive (you can see this both in the stage selector buttons and in the rocket design view). The correct behavior should be that "Booster stage 2" remains inactive, and "Booster stage" active, regardless of how they are moved in the design.

If you look at the activeness of the stage selector buttons, you can see that nothing changes; the last stage selector button is still inactive and the first and second one are still active.

Demo:

Issue already present in beta 4.