Does mayo "break"/decompose if you stop whisking then whisk the other way?
Welcome to the world of urban legends and old wives' tales.

Handmade mayonnaise can be a fickle thing to create if you don't work within the laws of physics and chemistry and don't achieve the desired emulsion. So like with other tricky processes, many "rules" have developed, that are more myth than method. (I was even told once to "always stir counter-clockwise"...)

If you think closely, it becomes obvious that an emulsion - for mayonnaise, tiny particles of oil suspended in water - is something completely different from tiny strands of something. One can not "unwind" or separate by stirring in the opposite direction.

My personal theory for the origin of the "don't change direction" rule is that if you constantly stir in one direction, you reach some kind of "flow state" and work in a steady rythm - the latter being far more important for a good whipping or stirring result than any direction. And that the rule predates the use of electric kitchen equipment, going back to the times where all whipping was done by hand.