I did a bit of research into this and it's probably not strictly a MTK-related problem, nor just ROG6D.
Users from other brands such as Samsung (S20 FE, S21, S22U, etc.) and Xiaomi (10) have reported the same issue with Netflix last year and this year. Netflix misidentified their devices with L3 instead of L1 widevine, thus no HDR for them even though DRM Info confirms that their devices are L1 and they are on Netflix's per-device whitelist.
The workaround for this Netflix HDR issue seems pretty simple: uninstall Netflix -> restart the device -> install Netflix again, and L1 should return.