Question HDMI Output stops when 3.5mm audio jack plugged in

Dec 1, 2021
Hello all,
Got a strange one for you.
Laptop, outputs HDMI > HDMI over Ethernet (this goes through the ceiling so is difficult to trace) > Projector. This works fine, no speakers on projector.
If I plug in a 3.5Mm audio jack into the laptop however, HDMI Output stops (nothing shown on projector, and output disappears from OS).
Tried with a different MAC & PC.
Tried a different HDMI over Ethernet.
Tried a USB Audio interface.
Tried a HDMI Audio extractor.
Ensured graphics software is on the highest performance mode.
Disabled HDMI Audio output device.
All exactly the same issue.
Short of changing our the HDMI & CAT5 cables, have you guys ever come across anything like this?
Any assistance is greatly appreciated.