Resolution Error - Tile is not square with current settings

Got a recurring error that is preventing me creating my terrain tiles using the batch option in 2019.1.
I've got a terrain to create that is 30000m wide by 36000m long and have heightmaps at 1024px square for each 1000msq tile.
The terrain toolbox is setup to show the overall size of the terrain as above which is then divided into 30 tiles on the x and 36 on the z.
The summary at the foot of the toolbox shows the tiles are square with a tile size of 1000m each axis.
I've reimported the RAW heightmaps and still get the same error even though the summary and setup clearly show the tiles are in fact square.

Any ideas on how to get around this error?
Thanks.