The Maxthon 2 data doesn't affect Maxthon 3 or 4.
I noticed, which is why I haven't cleaned it up (cleaning it has its own risks, obviously, e.g. %appdata%\Maxthon3 files are modified during updates).
a proper clean install will remove all your userdata along with guest settings
... And replace them with defaults, which all have google as the default -- which will be overwritten by my cloud-stored settings.
there is data that will get restored from the cloud
QED.
The default search for the address bar is set by the same one as for the search bar - Settings > Default search engine > Select your desired provider.
If that is set to the correct one and this occurs then it's quite likely a setting could corrupted some how, which is why it keeps reverting to the old one.
I would have thought it an inherited value, so that smacks of an error in a DLL or executable.
Of itself, it wouldn't be too annoying, but right-clicking the new-tab button also invokes the address-bar search function, so it has the same output.
To test this try step #8 in the Maxthon Troubleshooting & Tips and Tricks thread
That "fixes" it for the default user. Logging in resumes the problem.
Interestingly, the default-user ini file is modified during this process, but its CurrentSearchEngine value does not change (and if I change it manually, it changes back to google when the browser starts).
Using the portable version makes no difference.
So what it looks like is that I have to delete (at least parts of) my profile -- something that a clean install wouldn't help with.