I noticed that some MIDI control events randomly trigger the transpose bar in BASSMidi and it is really frustrating to constantly readjust it. I tried to find an option to turn it off or deactivate it in the settings but didn't find any. Is there a way to turn it off or does a version exist without it?
There is no controller bound to the transpose slider inside the plugin internally so it is 100% sure that random controller events cannot trigger the transpose change if they are not set externally.
Namely transpose itself is also exposed as a VST parameter (parameter 0) so you can associate it to any controllers inside your DAW (known as VST parameter automation, MIDI Learn etc.). So you should inspect your DAW if any controllers are associated to VST parameters.
It would be helpful if you shared what kind of DAW you are using and also share some more info about how your problem could be reproduced. You could also share your project file.