Done something wrong with a riverbank ... not sure what

In Osmarender:

Shows blue squares instead of the river.

Shows artefacts that are not there in the data (the teeth on the top river bank for example).

I have run it through a few checkers and JOSM validates … so what have I done wrong?

Maybe it is the same issue like here?

Add your areas there and ask for help if nobody knows a solution here …

There was couple of sections of reversed coastline near there, they might have been causing problems with the riverbank rendering. eg and
I’ve fixed both of those, though there might be more problems like that in the area.

How did you know it was reversed?

The artefacts, at least, have gone away. I think this is some sort of weird caching issue to do (maybe specifically) with coastline. It seemed as if the coastline nodes and ways were still being drawn even though they didn’t exist.