If someone can provide reliable steps to reproduce this issue, then we may be able to investigate further.
Open the Topology tab, select Advanced and click on the effect icon to open the Effect Layers dialogue.
Enter a name for the effect.
Change the turfx_density to 'Zero'. This means that no data space will be consumed for the density setting, so the Sensitivity control cannot be used to control the density. The default will be used instead.
Set the Constant value (immediately below the density setting, not the one below the height scale) to the required default density - 10 is suitable.
Leave all the other settings unchanged and click Apply and close the dialogue.
Select the Height Up tool and click anywhere on the baseboard.
What should happen is that the baseboard is painted with the effect within the paint circle at the default density. Instead, the whole route is painted with the effect.
Note that the effect spills over the edge of the baseboard.
Note also that the Height Down tool has no effect at any setting of the Sensitivity control.
Note also that any baseboard added to the map will also be painted with the effect.
Open the effect for editing and change the default density to a different value. Note how the density changes. So the application of the default value to the effect is working correctly. But there is no way to apply it selectively.
There does not seem to be any way to remove the effect other than deleting it and starting over. If turfx_density is set to 1-bit then
part of it can be removed by painting; it remains at the edges of the route (and, of course, that part that falls outside the edges). If the density is edited back to zero then it re-appears across the whole route.