Low RPM Reading

You don't say whether this is a new fault or whether it has always been like this. Is it a new or existing installation?

I see that there is an addendum to the EIS manual for Rotax 912 connection, see here:

https://grtavionics.com/media/912-M2000-4000-Addendum.pdf

The troubleshooting guide also has these words of wisdom:

Rotax 912/914 : On about 1 engine in 10, the tach reading from the dedicated tach output will be erratic. This is
cured by reversing the two tach output wires from the engine, or installed a 10k Ohm resistor (1/4 W or larger)
inline in the tachometer lead.

https://grtavionics.com/media/Troubleshooting-Guide.pdf

Many folks have had to install the resistor to cure erratic tach readings, sometimes after several hundred hours of trouble free use.