My 2¢:
I play in a weekly bar league game with a rotating dealer and 1 deck. No $, just points for your finishing place that accumulate through the season and gives access to certain games and bonuses at the year-end “big tourney” which does have cash payouts. All that to say it’s not the same as this, but relatable.
There is a HUGE variance in shuffling skills and I’ve certainly picked up on some patterns by some “dealers” who don’t do the standard casino gather/riffle/riffle/strip/riffle/box cut.
The 3 most common things that almost all rec league self-dealers do is that the riffle is (1) “chunky”, (2) they always lead the shuffle with the same hand, and (3) the deck is passed to the right for the cut.
So if you see the bottom card during the deck prep or bridge portion of the shuffle, it’s super easy to follow it.
Ex: King of Hearts on the bottom, dealer divides cards so bottom half is in left hand, left hand leads (drops the riffle first) = King of Hearts still on the bottom. Repeat 3 times = King of Hearts is still on the bottom!
Watch the cut (thick vs thin), and you can either tell for sure, or at least get a great idea, if the King of Hearts is in play, likely to come on the board/burn, or left in the stub at the end.
It’s a huge advantage to know this. I’ve stopped speaking out about it as it’s fallen on deaf ears (like the OP) but there are several decent, alert players at the tables who are clearly watching this and acting on that info.
So it may not be purposeful cheating by Player X, just crappy card handling skills with an obvious “tell” for card location that makes him exploitable by others, too… combined that with some jealousy of your skill set making him feel subconsciously threatened, and you get a Class 1 Asshole.
I’d be curious if he ALMOST ALWAYS leaves the lead shuffle card (from the same hand) on the bottom of the deck, no matter what card it is.
Like I said, just my 2¢.