Electrical

The multi-function switch is the logical suspect, especially if it has a "flash-to-pass" feature that flashes the high beams even when the head light switch is turned off.

This is not a common problem, so I would be ready to do some troubleshooting if the multi-function switch does not solve the problem. If it comes to that, you will need a voltmeter or test light, and have the covers off the switch so we can get at the wires to take voltage readings.

Also, look at the connectors for both switches. A pair of contacts inside a switch or a pair of terminals in the connector can overheat. The heat generated by one will migrate and overheat the other. If you see a pair of darkened terminals, and/or the plastic connector body is melted, that switch must be replaced, and the two terminals must be cut out of the connector, four inches of those wires must be replaced, then two new terminals must be soldered to the wires, then plugged in individually. The heat hardens that four inches of wire, and solder will not adhere to it. That heat can also cause the wires' insulation to melt, then multiple wires can short together. Watch for that when you inspect the wiring.