I have a rooted phone that has a interesting habit: it changes the MAC Address every time I log into a WLAN.

In the beginning it was funny, but now I want to stop it.
Do you know if there is any secret setting there I could take a look ?

It is a Android6 if you need more information please comment and I will update the question accordingly.

The Phone is a Cubot Max (Not Max2) 
and the WLAN-MAC-ADDRESS usually looks like 00:08:22:xx:xx:xx<BR>
the second part is random.<BR>

I read the MAC Address SETTINGS >> ABOUT THE PHONE >> STATUS

Kernel Version :3.18.19<BR>
jenkins@80-6 #1<BR>

Build Number: <BR>
CUBOT_MAX_v14_20170522

NO, there is no spoofing software on it. It seems that this is normal reaction of hackers in china, they do not like constant MAC Addresses.