Thanks, that makes sense. It does feel like a float error because of how inconsistent it is.
Unfortunately this didn't fix the problem. I made an extra column and set the quad to loop frames 0, 1, 2, and 3. Still looks the same as before.
I just tried a hard fix by decreasing the width of each arrow by 2px, so there's 1px of padding on each side. This fixes the problem (last time I just added padding on one side). That said, it would still be good to know what's causing the issue, if you manage to figure it out. Resizing sprites works for now, but it's not the most elegant.