I've been using the Contiguous Selection Tool to isolate regions for erasure, using the Eraser_hard brush_preset and I've noticed something puzzling.
Edit: I'm using the 4.1.0 pre-alpha but this happens in 3.3.3 as well.

In the Tool Options, if the Anti-Aliasing box is ticked then the selection is made but the Eraser_hard does not give hard erasure; it seems to be affected by the content of the selected region. If the Anti-aliasing box is not ticked then the Eraser_hard brush will totally erase the content of the region.

Example picture: https://imgur.com/a/dHlGl

The green arrow points to a region that was selected with Anti-aliasing not ticked. There is complete erasure inside it so the grey layer underneath it can be seen.
The red arrow points to a region selected with the Anti-aliasing ticked. The erasure is not 'hard' and seems to be affected by the image content. You can see where I made repeated use of the eraser tool on concentric areas on the lower leaf.

The original image is a .jpg photo and so it contains no alpha information that might be affecting this.

Can anyone explain why this happens?

The brush might be hard, but the mask might be soft. My guess is when you make a selection, it is making those edge pixels gray. Masks are grayscale images, not just solid black or white. Maybe that is it?

It is caused by selections in Krita being grayscale, so you can have semi-selected areas. The anti-aliasing on the contiguous select works a little funny because it doesn't anti-alias the boundary but instead selects areas that are sorta candidate for the selection in a semi-selected way.

You can see the semi-selected areas by toggling the alternative viewing mode for selections by clicking the icon in the status bar on the bottom left when you have a selection active. The marching ants only show up when a selected area is above a certain amount selected.
https://docs.krita.org/Selections#Display_Modes

Oh right. The alternative viewing mode does show this. There is 'mottling' inside the selected region when the anti-aliasing is ticked.

Thank you for your rapid replies