Does the even/odd count for diagonal moves reset every turn?

2014, using the optional rule where every odd-numbered diagonal movement costs 5 feet and every even-numbered one costs 10.

Does the even/odd count reset at the end of the turn or does it persist across turns for each creature? I've been assuming that it resets, but just had a situation where it made a big impact.

A player wanted to intentionally trigger a trap the party had discovered earlier because two fleeing opponents were standing in the hallway that was the trap's "danger zone". The opponents were clearly going to keep moving out of the hallway on their turn, they just hadn't made it through yet, so the player wanted to trigger the trap before the opponents' turns. The player was up in initiative, followed by another player, then the first of the two opponents. The nearest space that the player knew had a pressure plate was 50 feet "north" and 30 feet "east" of him. His move speed was 30.

Doing that in one turn would be 6 spaces of diagonal ("northeast") movement and 4 spaces of cardinal ("north") movement. So counting the cost of the diagonals out one-by-one, that's 5-15-20-30-35-45 northeast, then the cardinals make it 50-55-60-65 north, which is 5 feet too far for him to move on his turn with a Dash.

What he did instead was use the movement on his turn to move 3 spaces diagonally and 2 spaces cardinally, so 5-15-20 NE then 25-30 N. Then he readied an action to move the rest of the way as soon as his ally next in initiative moved or took an action. Since we had been operating under the assumption that even/odd counts resets on each turn, the cost for the second movement was the same as the first: 5-15-20 NE then 25-30 N, getting him to the destination.

All the players were excited by the clever move. I allowed it in this case but said I'd think about whether I needed to make a specific ruling on that for the future.

How do you rule that? It feels a little janky that using your action to ready a movement can get you farther than using your action to Dash. Is it worth making a special rule preventing that (like "even/odd counts persist between turns") or explicitly allowing it in the simpler case (like "you get one extra cheap diagonal move on your turn when you Dash") or is it niche enough that I should just allow it and move on?