Unfortunately I came across a location on my Edge 530 that has the same map visibility issue as I showed earlier. A whole part of europe is not ‘visible’ from top to bottom of europe. If I keep scrolling up or down, zoom in / out doesn’t make a difference 
I’ve uploaded the log files: https://files.mkgmap.org.uk/download/555/logfiles.zip. Description: ‘Logfiles for GerdP’. https://files.mkgmap.org.uk
Maybe the log files contain some useful information.
I can’t find anything obviously wrong in the logs. I see that you used different values for --max-nodes in the different splits, so maybe you repeated those tiles and didn’t clear the output directories? Even that should not hurt as long as you use the generated template.args file for mkgmap.
Make sure that you removed all the previously created gmapsupp from your device.
It might be caused by the small --max-nodes=16000 value for 4716. You have 6787 different tiles in those 10 mapsets. I don’t know the exact limits but I’ve heard that some devices have problems when there are too many.
If those hints don’t help please provide all the files produced by splitter except the *.pbf files.
It could indeed be the amount of tiles. I checked the garmin folder and it contained all 10 maps on device, and I’ve double checked the maps that are recognized by my Edge 530 and map 4716 was not recognized. I’ve increased the --max-nodes to 500000 for map 4716, ran the mkgmap command again and voila, the part that was missing from europe is back 
You are much more experienced on this, thank you for helping me to figure it out!!
Fine. The mapset 4716 contained 4857 tiles with --max-nodes=16000. So it is likely that the limit is 4096 (or 4095) tiles in one mapset.