This doesn't happen on desktop browsers so far from my testing, yet if I go to this website on an up to date Android device, the menu will close when I click into the 'location or postcode' box.
You can see the issue here: https://www.flyingbutler.com/
I thought that maybe setting onClick:{close:false} may have some effect on the issue but it doesn't. Can anyone assist ?
So I upgraded to the v9 codebase of mmenu and that did indeed resolve the problem. It wasn't what I really wanted to do as it required some other work to make the menu function exactly as before, but it did however resolve the issue this question was raised about.