Well it's a bug because it should be able to run reverse.
I must admit that on my side, as this feature was useless, i didn't test this. But I will do it ASAP.
I think that at the time your spindle is going faster than displayed value..
Well a bit of theory.
the plugin can only send frequency order to the VFD. For example , the plugin is going to tell the VFD to run at 76hz.
And how does the plugin get the frequency?....well it uses the famous PD144 parameter (depending on your motor)
if you tell the VFD that your motor run at 1440rpm@50hz, then the plugin is going to use this information to compute the order to send to the VFD...
well now in Mach3,you enter 8000 rpm.
The plugin computes the required frequncy: 8000/1440 * 50 = 278hz....so tells the VFD to run at 277,7hz...
the speed you see in Mach3 is the speed extracted from the VFD...but in reality, the VFD does the same as the plugin...it takes the frequency order and convert it to RPM thanks to the pd144 parameter. and something like 7997 is updated in mach3. (277,7*1440/50=7997...)
But do these values the reflect of reality? (most important thing!)
Well, now imagine that your spindle is in reality 3000rpm@50hz...we keep your current VFD configuration.
so when we want the spindle to run at 8000 rpm, well in reality it's running at 277,7*3000/50=16662! nearly twice!
That's why the pd144 parameter is very important in my plugin...
if you set Pd144 to 3000, we will notice that, for the same speed (8000) the spindle sounds to run lower...and i think this is the good parameter.
And finally, for the limitation, even if you do not check the pulley option, pulleys are always defined in mach3.
This kind of spindle can run from 3000rpm (50hz) to 24000rpm(400hz)...so you need to configure your mach3 pulley properly.
Hoping I have been clear.