Every time I have seen this using Indigo the Enable Analog Inputs option is set in the Configuration Register and the Analog Trip Points have not been set. The Analog Trip Points default to 000 for Off to On and 000 for On to Off. This results in Inputs 5 & 6 cycling between Off and On as the same analog value triggers both the On and Off indications. If the Inputs were in Digital mode the Inputs would not be cycling when connected to GND.
If you have access to a PC with a PLM or PLC such that you can run the free SimpleHomeNet Utility Suite it will display the Configuration Register settings and will also allow you to set the Analog Trip point values.
The topic below yours is related to using Indigo and that user was able to set the Analog Trip Point values using Indigo. I would suggest setting the Analog Trip points for both Input 5 & 6 as I believe they are still in Analog mode.
EDIT: forgot to answer your last question; this is not a problem with the EZIO6I.