Few questions ( don't know if you did any of this ) :
Q1. Did you check the Central Junction Box ( pass compartment ) for fuses #26 & #28 each 10 AMPs ?
Q2. Did you check the Battery Junction Box ( under the hood ) for fuse #3 30 AMPs ? I would suspect this to be good and functioning, as the high beams both are working.
Q3. Do you have any problems with the turn signals or brake lamps ?
Q3.1. I ask this, to start to check if there might be the start of a MFS problem.
If you look at the following diagrams, you will see that the BJB fuse #3 30 A feeds the MFS which directly feeds CJB Fuses #26 & #28, no relay, so the full current is via the MFS. The high beam activates CJB fuse # 16, 20 AMPs, to power both the high beam elements.
You can pull either of the CJB fuses #26 or #28, and see if power is on either of the pins, when the MFS is in the low beam postion, and the main headlamp switch is on. If not, and the high beams are working correctly, next is checking the MFS ( Multifunction switch aka turn signal lever ) for problems.
The problems with the MFS can be a bad connection under the steering wheel column, either partically unplugged or dirty / bent pins, causing no connection. This sometimes will manifest itself as intermitten turn signal functions, or problems with the L & R turn signals, where the CHMSL works fine.
Check the low beam fuses, prior to digging into the MFS connector or switch itself.
If you want to search on Multi Function Switch or Multifunction switch, you wil find quite a few posts on the topic.
good luck, let us know what you find.