my suspicion is that the low beam turns off because Yamaha used the same switchgear as on the X/R which have an H4 headlamp where it switches between two filaments in the same bulb.

i've looked into this myself for the same reason - you need to download the XTZ service manual (become a supporter and you can do this from this site) and that contains the wiring diagram. You'll need to run a new feed to the dip beam lamp to avoid overloading the wiring. I would suggest running the new feed all the way to the back of the lamp and just leaving the original plug unplugged, that way the hi-lo selector will turn the high beam on/off and the dip stays on. Also, done this way it's easy to undo if you decide to sell the bike and the wiring harness hasn't been messed with.