Hi welcome to our forum. This sounds as if it could be down to some unwanted video processing in the TV or perhaps a displaced hdmi cable. On most TV’s the adjustments are input specific so it might not affect the TV tuners or even simply connecting the Humax to a different hdmi port on the TV may fix it.
Some suggestions
Push the hdmi cable firmly into the TV and Humax.
Set the output of the Humax to 1080i
Move the hdmi cable to a different hdmi port on the TV.
Try a different HDMI cable.
Turn any video processing options on the TV off while viewing the Humax.
Knowing make and model of TV would help (If a online manual is available).