Incidentally the zooming worked perfectly on my Moto G (3rd gen) with Android 5. (5.0.2, I think).
Since I switched to Android 6, I've also noted another issue possibly related to zoom. On my largest map by file size which is a multi-scale set, when I select the 1:25,000 map set it opens the 1:50,000 map if it has previously been set below the 1:25,000 scale although the label that appears briefly indicates it should be the 1:25,000 map. A click on the + zoom button then brings up the 1:25,000 map. This wasn't happening on Android 5. Not much of a problem to me but strange.
Thanks for your help.