MS Basic has a mono reverb even if Audio FX is set to No effect
1) Open any recent development version of MuseScore 4.0. I tested with both Beta and Release Candidate 1.
2) Create a score
3) Add some notes
There is an awful mono reverb in apparently every MS Basic instrument. In MuseScore 3.6.2, there is the Zita reverb, which is a better quality stereo reverb and you can deactivate it in the synthesizer if you want to use your own reverb or simply don't want any reverb.