layer glitch with mbtiles overlay

[CSIG]

I create charts for the Inland Waterway of Ireland for the IWAI.
We use the mbtiles format for the charts so they can be used on Android, IOS and Windows based devices.
I want to give users the option to choose between Imperial and Metric spot depths without having to produce two sets of charts.
In Android and IOS apps, when using a second mbtiles file (with teh spot depths) as an overlay the depths display correctly in line with the zoom level selected but with OpenCPN the display is garbled because it seems to display more than one level of the spot depths at any time.
I believe it is because the spot depth files have a transparent background which is required for the base map to show through.
Is this a bug, or is there a configuration change we can make to fix this?
I will upload an image to show this shortly.
best regards,
Brian.

[CSIG]

OpenCPN image

[CSIG]

Correctly displayed in Android app.