clone command seems to be hitting imaginary world boundaries

created world that I've been creating in for a few weeks on an imported small flat map. Tried cloning something 290 blocks away:

"/clone ~-291 ~ ~1 ~-291 ~ ~11 ~ ~ ~1"

It gives back:
Cannot access blocks outside of world

If I adjust that to -251, it works.

I thought I was hitting a worldborder, but I don't think that is true.

Not sure what else I can do to help troubleshoot this. Let me know.