Usable modules enabled by reactor chambers such as:
- Thrust Blast
- Reactor Boost
- Anti Gravity
are still on your hotbar (if you assigned them) after you switch to a reactor that does not have the chambers to allow them.
The modules still charge but have no functionality, so it's only a minor GUI issue.
If you switch back to the reactor that offers these modules, it would be nice if that re-appeared on its previous slot of your hotbar, provided it's not taken now.