In my opinion as a GM who likes to skip a rule here or there if it would make more sense story-wise, I feel the most important consideration is managing expectations.
You, as a player who relies on rules to win the combat, expect these rules to hold, so that the goblin can't just get out of a flank like that. The GM expects the combat to be flashy, and that goblin was supposed to be the one that runs away screaming for his mommy, who turns out to be a giant spider that will come as reinforcement. Or maybe the GM is simply overwhelmed by all the rules, and merely forgets them at times.
What I suggest you to do is talk to the GM about rules philosopy. Tell him that you need to know which combat rules you can rely on so that you can plan ahead your moves and deal maximum amounts of damage to those goblins, which for you is an important factor in enjoying the game. Find out why your GM doesn't stick to the rules, and find a way to compromise. If all of you like rulesy combat, you can be the "rules narrator", who tells what is happening in terms of combat advatages/disadvantages when the goblins move around, as a way to assist the GM. Maybe the GM can give you a list of rules that will always hold (and where you are invited to complain if they're violated), and a list that the GM thinks is more fluid (hopefully for some good reason).
In short: Talk to your GM to let them know that you like the rules, and why you like them, and then listen to the GM as they explain why they do things differently, with the goal of finding a compromise that is acceptable to both sides.