Well it certainly is not locking as it should.
There are 2 maybe 3 kinds of quick connects used by different manufacturers, they often says they are compatible, but I would say probably not a 100% match up.
Do you remember the name of the manufacturer of the slave that is in there now?
You said you replaced master at the same time, I assume this came as a master/slave kit?
And who is the manufacturer?
I am thinking there is a mix and match issue, because as said above this has never been an issue with Rangers, hard to bleed....yes, hard to get quick disconnect to release.....yes, quick disconnect popping off....no.
Shops that work on hydraulic systems may have similar connectors, and they could check the slave style connector and put a new, matching, end on the master line, not sure on that though.
But at least they could tell you why it keeps popping off and may offer another solution to lock it in place.