Help me diagnose the cause of this intake valve burn

I would like some help diagnosing the probably cause of this intake valve burn as seen in the picture.

167fmm engine.
229cc OHV push rod type.

Now here is the rub.
this is engine originally 9:1 CR, I have decked the head a total of 35 mill to increase the compression to a calculated 10:1 CR.

Of course i am aware of the increased compression brings higher heat, but running 93 octane, and never experiencing a pre ignition engine knock, i did not/do not anticipate a condition that caused a burn while the intake valve was extended. And rather would expect a burn to occur more on the exhaust valve if due to the higher CR.

But note it was a slow failure as i noticed the valve reducing in diameter as it rotated and burnt away during operation, receding deeper and deeper into the valve seat causing the first manifestation adjustment the valve lash screw higher and higher until there was no more thread for adjustment.

i measured flatness of the head and the rocker assembly, they were fine so i knew it was not a warp or valve train problem.
The final failure of course total loss of compression as the large pit obviously didn’t allow seal.

My research indicates a burnt intake valve generally due to valve leak, where the hot exhaust gases might slip through and pit the valve.

So Id like any info that might be available.

I should note just prior to disassembly i noticed the valve stem seal had slipped up and off the stem, this perhaps merely indication of excessive pressure due to higher heat.