There is little magnetic field from even a twin unscreened mains cable. The live and neutral return currents being opposite work similarly to a twisted pair network cable and largely cancel any external 50Hz electromagnetic radiation. . In any case the frequencies involved in HDMI interconnects are way higher than the main at 50Hz,Firstly , no cable longer than 8 meters has ever passed the high speed test suite ( category 2 ), however , the high speed test suite has always tested to 4096 x 2140 ar 60 fps ....so assuming it fails at some point between 1080p and that early assumption of 4K, a longer cable may well be fine for 1080p....cables as long as 15 meters have worked for 1080p...they just wont ever have passed and gotten a genuine certification so there are no guarantees.
Second, general rule of thumb is never run signal cables beside mains cables.
That is a rule everywhere.
The magnetic field from a mains cable is of a strength that can swamp any signal carrying cable.
Having said that, HDMI cables built to spec have a complete metallic outer shield that should be able to handle it ( faraday cage etc.)
Its still good advice not to do it though.
Seen this many times, never experienced any issues at all, unlike poorly screened coax interconnects carrying UHF in close proximity to HDMI cabling.
All the regs regarding seperation are purely down to electrical seperation from medium voltage cabling and low voltage cabling. A plastic screen is fine which clearly has no effect at all on induced voltages. It's purely based on on cable damage like for say drilling through both cables buried in a wall.
I speak as a ex National grid Engineerof 40 yrs experience. Even a unbalanced 400KV overhead line of many miles carrying up 4000MW, only requires the transposition of the conductors to cancel the unbalanced induced currents over a very long distance, while 3 phase cables require none.
So sorry your rule of thumb has no foundation at all, it has no foundation in any electrical sense.
A fully screened HDMI cable is not a Faraday cage at all. Electromagnetic radiation will pass through it unchanged. Only a screen of a ferric based composition as used in a transformer core will have any effect all. Basically a transformer.
A Faraday cage has no relevance at all. It merely reflects that within a metallic box that all internal points are at the same potential. If you are in a Faraday cage (Aircraft - Car, that is struck by lightning), then whatever you are touching is at the same voltage so no current passes through your body - so you feel nothing. Birds sit on high voltage overhead lines, if they happen to peck on a earthed point they die instantly. Voltage isn't the issue it's the current that passes through your body that can kill you.
It has absolutely nothing to do with external induced inteference.
Incidentally my car is a Faraday cage as everyone's ones is.
Like everyone else my 4G phone works just fine.
Please explain how your theory explains this.