How are your Button Widgets setup… PUSH or SWITCH
The way a Button Widget works is that each time you ether press or release it (regardless of above modes) it will call the assigned BLYNK_WRITE() function… so if your button sends a HIGH signal, then the BLE connection drops, the motors have already been set to run and will keep doing so until another button action is detected setting the function LOW (and thus stopping the motors… of course resetting the device will also reset the digital pins and thus stop the motors 
So you should really have it coded that the motors ONLY run when seeing a continuous loop of HIGH signals from the Buttons/Function with default settings of OFF so that in case of signal loss, the motors stop. Search this forum for keywords like Fail Safe and Rover for interesting reading 
Check out a simple control I used for my Rover… Set for WiFi, but that is just the Blynk connection, the Button logic will be the same for BLE.
EDIT - I looked at my code again and realised I never got around to implementing my own final failsafe in it 
But basically now it would be a timed App connection check every few seconds or less, followed by an auto stop (if disconnected).