Ok so I returned the first one that did not stay secured and the replacement from the other retailer arrived, it fits perfectly and stays in place as expected.
So if you get an Oplab module that does not fit then exchange it as it definitely seems that some are faulty.
I think the problem with the first one was related to the catch not being strong enough and the Oplab casing being slightly out of tolerance, this would cause the module to pop out by about 0.5mm after securing the catch/locking tab, the module would not show up in the app and one of the LEDs showed red, applying pressure to hold the module in place would cause it to work but only while the pressure was applied, then it would disconnect and pop out.
The new one fits much better and even when the catch is open it still stays in place.
Hope this helps anyone having the issue, certainly would not recommend filing or modifying the Oplab module as it would probably void the warranty.