My guess is drought and bark beetles in combination causing mortality.
Yes, the chisel tooth chains cut quicker than the chipper tooth but they also become dull much faster. They work best on larger displacement saws (70cc+).
I am getting conflicting info on the H51 Husquavarna blade
Website says
A high-performance, low-vibration chain with square-corner "chisel" cutters for high-production cutting of clean wood. Round ground cutters an standard sequence. Recommended for professional use on 50cc to 100cc saws. (Description shows: # drive links / pitch / gauge)
Anothner official Husky site says
The corner of the cutters on a semi-chisel saw chain are slightly rounded. This helps the cutter keep its sharpness longer, especially when cutting hard or frozen wood, or when working in dirty surroundings, where a full chisel saw chain would quickly lose its sharpness.