Have you tried opening the widget with data or a dummy 'tethered' connection? With my device this issue no longer seems to occur for new mbtile layers that I add to the device. I don't place the file(s) in a subdirectory.
I am uncertain whether turning on data / tethering 'fixed' the issue, but I have just got into the habit of checking the mbtile layer whilst still connected to my wifi before going into the field, so may have just got a work around rather than fixed it.