I've started to convert the setText to the performance-friendlier updateText and setTextFast methods - however I added these to the canvas API just two days ago, so you need a *really* fresh FGData (you can just copy Nasal/canvas/api.nas from the repo if you don't want to reinstall everything.
https://sourceforge.net/p/flightgear/fg ... as/api.nas
If that's the correct diagnosis, there should be an error message that enableUpdate (or enableFast, whichever is encountered first) is not an available method, and the resulting Nasal runtime error prevents the whole MDU framework from loading, at which point you get to see the bare SVG with all layers on top of each other.
The good part is that this will save tons of property I/O and allow a faster MDU update cycle.