IT IS a GROUND problem in the cluster.
What happens?
The dash lighting, the turn indicators, and the high beam indicator are all grounded to the cluster ground plane. If it is poorly grounded, turning on other lighting raises the ground plane above ground, and reduces the ground path for (in this case) the indicators.
'I bet" if you were to turn on the high beams and play with the dimmer, you would also see the high beam indicator change brightness