The filter takes effect in the editor view if the nodes are selected.
The dialog of the filter doesn’t accept a negative value of the radius.
(Rounded Corners Filter Outside/Inside, it said a negative value is acceptable)
In Edit view, if you want to round all, you can either have no selection or all selected. If it does not work, then probably some small thing was selected accidentally.
I don’t know if this is due to the glyphs or the fonts. I can tell it won’t work properly for each of the fonts I am working on. But at least it works with Android-Clock.ttf.
For now, I think the filter doesn’t work properly with both of the editing and export modes. To clarify this issue, I captured the screens.