Once airborne, any aircraft pilot can encounter this bug.
While flying, open any dialog (place a marker on the map, or press ESC while in multiplayer,..) and you'll find your aircraft's throttle has dropped to zero for no apparent reason.
This is especially dangerous if you're flying low, or at a certain speed, the aircraft will quickly drop its nose and crash.
Repro video : https://youtu.be/8oSoaZpxENU
OR A MORE VISUAL REPRO :
Repro video : https://youtu.be/o2X87SZ8KI8
There seems to be a workaround, though it is really annoying.
Hold the Throttle Up or Throttle Down while opening the dialog.
In the case of the ESC menu, hold Throttle Up and press ESC. Once the ESC menu opened you can release the Throttle Up key, and the helo will maintain its altitude.