Murray, thank your for your reply. As I found a workaround already (defining PlotRange explicitely, the middle plot), I posted my question rather out of curiosity what is going on under the hood of RegionPlot. I had found indeed a StackExchange post on this issue (https://mathematica.stackexchange.com/questions/156467/extra-edges-added-in-regionplot). Some comments there recommended increasing PlotPoints and/or MaxRecursion levels in RegionPlot, which I had already tried before and apparently do not resolve this behaviour at all. A helpful suggestion was to add Option Mesh->All to RegionPlot to show the grid used to discretize the region for plotting. As I suspected, RegionPlot appears to create different meshes depending on how the region was defined and how its PlotRange option was set; see below. I did not find this mentioned in the documentation. Might be helpful to add an explanation there.

And here's an EVEN SIMPLER example. (Still Mathematic 12.2 on Windows 10)

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regionPlot simpl...nb