That's strange, as we are still limiting individual tracks to ~21k points, as before - in fact, this part of the code hasn't really changed since v1.8.
Have you done a round-trip to the unit with this data? If not, how are you exporting it?
Here is an example that I have just tried, where isobaths need more than a single HB slot:
I wonder if the fact that the track name is the same is causing issues after being put on the SOLIX, although this has not been an issue in the past as far as I am aware.