So, this question is kind of the driving wheel behind several other recent questions of mine. It has been partially and/or indirectly answered elsewhere, but I'd like this question to centralize the full answer.
Basically, I recently played with a DM who, although adds a nice deal of storytelling to existing Adventurers League modules (which enhances play), also has the habit of suddenly changing existing rules within game sessions. On several occasions, these changes directly impeded the usefulness of some characters' features, and when I told him about the existing rules, he either discarded what I had to say or decided to rule otherwise anyway. (Non-exhaustive) examples are :
Made net-restrained foe attack their net without the disadvantage caused by the net (which led to the Multiattacking enemy to free themselves easier than the normal rules would allow them, thus being able to make their other attacks in their turn) -- this is incorrect according to this question : Do you have disadvantage at attacking the net that's restraining you?
Made a target of a successful Flee Command word Disengage before moving away instead of Dashing (as per the default rules, the target should have Dashed, thus getting opportunity attacks in the way, which the player was counting on when using the Command spell) -- this is incorrect according to this question : Is the target of a successful Flee Command able to Disengage before moving away?
Made us spend twice as much movement points to travel diagonally on a square grid map (which is even more restrictive than the "realistic movement" from the DMG where you alternatively spend 5 and 10ft when moving diagonally)
Prevented the use of a Magic Item, firstly because the player had not mentioned it before play (but was not asked to do so and is not forced to do so to benefit from it when not prompted according to this question : Must the DM be told before a game what magic items a PC owns?), but later said that even if the player had mentioned it, he still would have refused the magic item's usage because his sessions are connected (the sessions in question were parts of a module trilogy, specifically DDAL07-07 and DDAL07-08) -- this is incorrect according to this question (case 1) : Can an AL DM prevent you from getting access to a Magic item obtained between two of his sessions?
When you're playing in a homebrew game, the DM is of course empowered to make all the rule changes he wants. But I feel like in Adventurers League organized play, a DM is not allowed to change existing ("core") rules. I might be mistaken, hence why I'm asking this question.
So the bottom line question is : is an AL DM allowed to change existing rules? If so, to what extent? If not, what consequences should apply when he does so regardless?
Note that, although the actions of this DM have left me quite angry (it's a flaw I have, unfortunately), I don't believe he is a bad DM per say - quite the contrary, in fact. His storytelling skills are, I would say, better than mine (I am also an occasional DM), and it frustrates me that I'm unable to enjoy his story-enhanced games because of these rule changes he makes that impede the strategies we try to apply during play. I want to have an official, clear answer to give him that he should not keep changing existing rules going forward, in order for all his players to enjoy his games like they certainly would otherwise.