I read a used bike test of XTR 660 & F650GS a while ago(2008).
There was a mention of some XT's being bulb blowers but there was no explanation for it.
What year is your bike. Have you asked Yamaha dealer or technical support?
Philips do heady duty bulbs which are vibration resistant as well as one called motovision.
As already mentioned, high constant voltage or spike will reduce the bulb light. A blown bulb is not necessarily blackened: you will see a broken filament.
I changed the bulb in my 2009 XT at 1000 miles because of brightness during winter months with Halford 50%+ (which is actually GE). It is still fine