Level Order is important to consider when Geo-converting, although in my limited experience hasn't caused spikes rather omissions instead , and may be causing a conflict. I heard and read stuff and my thinking is that the different meshes work like orthoes with the layer underneath fill the space of masked areas.
Im guessing
Default mesh Level7
DLC Level 10
Your Under layer mesh Level 11 for 20m SRMT for example
And the high detail meshes Level12-14 for 2 metre mesh
So if your High Res map was geoconverted at level 10 the same as the DLC in my example, could be trouble and worth checking out I think
Another issue I can see is masking with two non connect areas sharing the same level 12 cell. So if you have a level 12 cell with map data on both sides with a gap in the middle is when you will have a masking problem with one side disappearing. But I haven't seen spikes from this.
When you download your file for your favorite area it is not always magically ready as in the tutorials.
Some time you got to make them ready to geoconvert. With QGIS
Luckily we have QGIS and in the coming weeks I will post on my methods and work flow for making the Sydney mesh Project thingie for others to digest that will help the communities knowledge base on elevation mapping. Rather than take individual requests, just no time for that but such a walk through can increase the communities knowledge of QGIS and some of its benefits.
Be sure to read the fine elevation tutorials on the forums, as I won't be covering much that has already been covered so superbly so you will need to study up on that.
Rather it will focus on preparing your map when its not ready to geo-convert using the Sydney project a an example. Because when I started it definitely was not ready with spikes galore.
More stuff incoming.