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The 817/818 is a very reliable radio when it comes to external keying and one of you has confirmed the same behaviour when using a different radio. From the descriptions it sounds like RF getting into the wrong places in the amplifier. This could be RF radiated?inside the enclosure from the PA. Could be RF getting onto?the external PTT line, could be RF getting into the supply of logic and switching circuits.

Anaxi said he is waiting for a FW update - that's a strange troubleshooting approach but maybe it has something to do with the RF sensing circuit.?

I would terminate the amplifier into a dummy load, make up a very short PTT cable (ground centre pin for TX) then try to TX from your radio with the lowest power possible - CW or FM is most convenient for testing. The point of doing this is to exclude the PTT cable as a possible cause. I couldn't find a manual but it looks like the amp has some kind of auto band select. Can you manually select the correct?band rather than relying on the RF sense?

I would use a 'scope to look for RF in places it shouldn't be such as the logic and switching circuits of the amplifier. Be prepared to put some 10 nano or even 100 nano capacitors to ground in areas where you think there should not be RF. This is not a 3 minute task, and requires persistence.