Have a look at the code:
The grids module can operate in two modes normal mode and euclidien sequencer mode. The external also allows this, but whereas the physical module reuses the same inputs, the external provides three more. You set the mode by sending a negative number to first inlet (look it up in the code). Using both sets of inlets for the two modes at once may result in unspecified behavior, since that is not possible with the physical module.
I admit I wasn’t terribly interested in the euclidien mode and I don’t remember testing it extensively, so there may also be problems with it. In any case, implementing a euclidien sequencer as an external is overkill. It would a good patching exercise to patch one together in pd.