Hriston
Dungeon Master of Middle-earth (He/him)
There is an optional rule, a rule of which you are completely aware, that gives advantage to attack an opponent that you are flanking.
If I ask if you are using any optional rules in general, or ask if you are using the optional flanking rule in particular, and you say that you are not, I'd take that at face value.
Then, during play, you consistently give advantage for flanking, while still claiming that you are not using the optional flanking rule but just using your DM authority to judge things on the fly, but your judgement just happens to perfectly match the optional flanking rule every single time, can you understand why I'd accuse you of lying to my face?
Yes, I think that would be dishonest.