This issue is being looked into as we have had reports on no depth for the trackpoints if you have more than 1,000 points in a particular track. Your issue though may be software version related as I cannot find in any documentation as to when the depth data was added to each trackpoint – and the last software released for the Matrix 37 was the 3.840 version back in July of 2007. So your unit may not even be able to run a new enough version of software to even have the water depth recorded along with the trackpoints.
Since h:depth is a standard Humminbird extension to GPX one would think that data supplied under that Humminbird defined extension would attach to every GPX element that declared h:depth, i.e EVERY waypoint and EVERY trackpoint.
ELSE, Why declare the h:depth extension at all?
Since the fact of the matter is that we have no evidence that any version of any Humminbird firmware fully complies - or ever will - why should those who need this data not go to Lowrance/Eagle who have no problem connecting every waypoint, every trackpoint, and every fish arch with valid timestamped and accessable depth data?
I thought the purpose of a "fishfinder/chartplotter/trackplotter" was to plot actual depths (structure) and suspended objects (FISH) to real world coordinates. Perhaps this whole concept of indecipherable SI images unrelated to real world coordinates has replaced the delivery of data that is usable in real world situations with pretty, but useless, graphics.