The headlamp flash is on a dedicated circuit with its own relay so not surprising it works when dip doesn't.
Suspect the dreaded TSM module which interprets stalk actions for indicators and headlamps. This unit then provides the switching signal to the dip/main relay on orange/blue cable. The dip output from relay then runs to fuse 17 via blue/pink cable and on from fuse to each headlamp on pair of blue/red cables. If main beam is working normally it's safe to assume the main power to dip relay (two blue cables) is OK.
The dip/main relay is wired for dip failsafe. Strangely this suggests the TSM module is providing a permanent main beam switch signal. You could prove this by interrupting the orange/blue feed to relay (if possible) at which point dip should come on.
