slant 6 problem

I once put a brand new set of plugs in an old Sunfire. And a few days later she lost a cylinder. I isolated the bad cylinder, pulled the plug out, and
"it looked fine".
But I'd been fooled a time or two in 52 years of working on cars, so I pulled a different plug out, and it too
"looked fine".
So I swapped them around, and now the dead cylinder followed the "fine looking plug".
So I put a brand new " fine looking plug" in there and now had 4 cylinders again.
So then I went and studied that no good, but "fine-looking" plug. And with a magnifying glass I found a hairline crack in the porcelain. That "fine-looking" plug ran perfectly for a few days and then just like that, it cracked the poecelain and that "fine-looking" plug was 100% garbage .
Two things I have learned in my life;
1) never trust a fart, and
2) never overlook the obvious.

IDK if your problem is a plug or not, but if you isolate the bad cylinder, and swap a different plug in there and the problem goes away; then,I'd say that is all the proof I need.
And that just leaves the why of it.
In my case it was detonation, because it later cracked other plugs. actually broke them,and swallowed the pieces.
As for your slanty, IDK.

BTW, not trying to be a jerk, just saying you can't always eyeball a plug and know that it's fine. I did not comment earlier, because there's always a chance that you were an ace-mechanic and knew what fine meant. So I didn't want to embarrass myself too early in the thread,lol.
I like to say things like ;
"they looked fine to me", or;
"since they're brand new, I assumed they were fine."
That way it doesn't hurt as much when the Master Mechanic says " what's a fine plug look like?"
Then I can answer, "What do you mean", and nobody gets their feathers ruffled.
Happy Hunting