As for your issue… there have been a few topics about things like Fail Safe routines and connection checks… basically a necessity for anything that has potential to run amok once off it’s leash.
The key is to write your code so that the default state is do nothing, stop motors, or otherwise safe action… then ONLY proceed if certain conditions are correct i.e. confirmed connection, confirmed reception of control commands, even a built in timer saying “if nothing has said otherwise in the last 5-10 seconds, then shut down!”… and so on.
Look at the various ways Blynk offers for connection management here…
And here is one way of keeping your code running (and thus able to apply the fail safe logic) even after disconnection from Blynk…