We discussed the issue extensively in the Swiss IRC [1]:  @SimonPoole mentioned that 

> the problem is the old thing that the historic boundaries along the lake and the lake itself are mixed up, and while [he] explicitly checked the old boundaries, [he] might have unglued a node of the lake accidentially

The issue is fixed, the tiles should automatically be regenerated and show up when you flush your cache.

[1]: https://osm.ch/hilfe.html