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Do you wait in the boot.py until the connection is established, or simply use wlan.connect(...) and then end the file?
As far as I know, the statement you mention should be true, and the connection you made in the boot.py file should hold.
You should be able to test this though, through using
from network import WLAN
wlan = WLAN()
print(wlan.isconnected())
in the main.py to test if we are connected to a wifi network. If this returns false, there is no connection. This should also hold True if the connection was created in previous 'session' that you ctrl+c'd out of, which is, to my understanding, very similar to the transfer between boot.py and main.py.