It's my first time with FLI, so maybe I am doing something the mod is not designed for.
I am filling a tank in Exotic Industries with Lubricant and have a straight and a normal version of the FLI connected directly to the tank.
The straight one immediately displays a value and is configured to "Fluid level mode: High ideal" (what ever that is, it does not seem to be documented anywhere). For the other one, the indicator type is not configured, but even if I changed it to High ideal as well, it stays black.

https://derpumu.de/img/FluidLevelIndicatorNoDisplay.png

Thanks for letting me know, it was a bug indeed.
It should be fixed now!

Thanks, I'll try it asap. Could we get a line about the meaning of High/Mid/Low ideal in the mod description page?

Sure, it is added now :)

Thanks a lot! I also can confirm the display works now on both indicators.

Great, thanks for confirming!