I, along with one other person, have found bugs when toggling this feature.

To reproduce my own issue regarding the feature, perform the following steps (may be dependent on other fundamental data which i was unable to gather at the time):
Open LMMS with the "One instrument track window mode" setting on (located in the first tab of the Settings window)
Open an instrument track window
Turn the aforementioned setting off
Attempt to open a different instrument track window
LMMS will then crash

@tresf has also observed bugs with this feature.

Operating system: Windows 10
LMMS Version: 1.2.0 rc3