Hi Max
This functionality is currently still in beta under the shift light functionality (vehicle functions sub menu), we've been revising this and will most likely remove the option for up and down indicator on the gear display, as it really just doesn't make sense why you'd want to have a down indicator, and we've had the same behaviour you described - when enabling both, it doesn't display any arrows, which supports the point of it being impractical.
If you're unable to locate the menu, send through a full autoscan to our support desk: support@hexgs911.com
Kind regards and safe travels
Ruan