How to stop MAC address from changing after disconnecting?

Network-Manager will reset your mac address during the wifi scanning.

To permanently change your mac address:

Edit your /etc/NetworkManager/NetworkManager.conf as follows:

[main]
plugins=ifupdown,keyfile

[ifupdown]
managed=false

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

[keyfile]

Edit your /etc/network/interfaces by adding the following line:

pre-up ifconfig wlp68s0b1 hw ether xx:xx:xx:yy:yy:yy

The xx:xx:xx:yy:yy:yy is the new mac address obtained from the output of macchanger -A wlp68s0b1.

Reboot and verify your settings.

Arch-linux wiki : Configuring MAC Address Randomization

Randomization during Wi-Fi scanning is enabled by default, but it may be disabled by adding the following lines to /etc/NetworkManager/NetworkManager.conf or a dedicated configuration file under /etc/NetworkManager/conf.d.

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

Setting it to yes results in a randomly generated MAC address being used when probing for wireless networks.

macchanger when first installed gives you the option of automatically changing MAC address with every new connection. since I don't see the option on the --help page I would suggest uninstall and reinstall. The Yes/No option will crop up asking if you want it to randomize etc etc. Just select 'No', you should have smooth sailing from there.