Azure Aearch indexer: ‘Documents succeeded’ keeps increasing despite high watermark detection policy

I have set up an indexer for azure search with azure sql as source, using High Water Mark Change Detection policy. I have run an initial index that resulted in all rows being added, which is as expected.

When I subsequently make changes to to rows in sql and run the indexer, the updates are reflected. Which is also as expected.

However when I run the indexer again it seems the updated rows are always being re-indexed, despite there having been no changes since last index-run. This leads to the number of rows being indexed increasing steadily:

This is worrisome because I will eventually have millions of rows and constant updates affecting those.

I was under the impression that I could use the High watermark method to avoid indexing all rows every time, but somehow it does not appear to remember the high watermark of the most recent index-run. Have I made a mistake in my configuration?