mac address of raspberry pi keeps changing

Cause:  NetworkManager can randomize your mac
Solution: 
sudo touch /etc/NetworkManager/conf.d/100-disable-wifi-mac-randomization.conf
[Paste the following lines in "100-disable-wifi-mac-randomization.conf" and save]

[connection]
wifi.mac-address-randomization=1

[device]
wifi.scan-rand-mac-address=no