So I was playing on my testing map ...
rambling about the map
... and changed one shore to be flat for my nuclear setup ...So I had changed the fill size to 1x1 and was running down "repainting" a one wide strip before filling the rest.
Then I came close to another ... indent(?)*³ ... of said lake ... and suddenly I dragged that shore along. (I do have saves short after and long time before ... which I will provide if there is interest)
Pictures
As you can see (1. Picture) ... I dragged a square of land with me including water ... (that means where the shore was closest to my bridge got "overritten" while I was moving down)
The 2nd Picture shows where it all started. (but after it had happened)
The 3rd and 4th Picture shows where it did happen on map view ... to show where it was related to my spawn area.
Mapexchange string
I do not know if this is a one time event *** ... or even if it can be reproduced ... I did not do any further investigation of this.______________________________________________________________________________________
Footnotes rambling
