I'm so sorry. I got it wrong. Threshold should be more than 0.000.
Merge by Distance completely works.

I tested both rB7fbd9b67afa and rBbf49bb354fca, and they have the same issue.
The results of sculpting with subdivide modifier were like the video capture attached above.

@Daniel Martinez Lara (pepeland)
Some similar issues, T75653 and T75654, were reported and then merged into T75566.

I tested D7404 and it works pretty well in both wintab and ink.
It perfectly fixed this issue.
Thank you so much, @Nicholas Rishel (nicholas_rishel)!

@Nicholas Rishel (nicholas_rishel)
This one

Krita has an option called Use mouse events for right- and middle-clicks in its configure.
If I use this workaround, side switches work in my environment.
But the option is relatively new, which has been implemented since ver 4.2.
And I didn't know 'Hover Click' is not a common thing.
I gradually understand what @Nicholas Rishel (nicholas_rishel) is saying at D6675, and this issue is not a bug.
Thank you for looking into it.

I found yet another fact...
When I use Windows Ink at the blender preferences, the pressure doesn't work.
So, I enabled Windows Ink at the wacom driver settings, and then the pressure comes back again.
BUT, in turn, Hover Click breaks again if I do it.

I found another fact.
When I changed Preferences>Input>Tablet>Tablet API from Automatic to Windows Ink, everything worked.
Hover Click came back.

I found that D6675 is closely related.
I guess Hover Click has broken in my environment.
Hover Click is one of the pen button modes.

The button name just comes from https://docs.blender.org/manual/en/dev/grease_pencil/selecting.html
Is this excellent one removed in the future? If so, I'm sad...

I guess some changes that were done between rBa24f52c51cff and rBb555b8dedce0 have fixed this issue.

From a user viewpoint, it becomes hard light mode when there's something white behind the GP object.
And it becomes overlay mode when there's something black behind the GP object. It's not only a matter of GP layers, as far as I know...

In T75190#900036, @n1729 (n1729) wrote:

So, it means that Overlay mode is actually Hard Light mode here.
I'm sure that a certain process is reversed somewhere.

@Clément Foucault (fclem)
Thank you for your kind explanation.
I didn't notice Value 0.5 is no longer RGB 0.5, which were the same in 2.82.

Ver2.83 seems to take into account the world color, so I think the correct result is expected to be the following image.

Overlay blend mode is very important to the grayscale to color digital painting process.
And I can't understand the detailed implementation, but at least the result of ver2.82 is correct.
If the result of ver2.83 was correct, many artists would have nightmares.
Any other painting software has the same result as ver2.82

Thank you, @Antonio Vazquez (antoniov)

Duplicated? T69965

I builded and tested c971e812d5e2 and the earlier bf1b323b1544.
As @Antonio Vazquez (antoniov) mentioned, I confirmed that this issue begun from c971e812d5e2 at least in my environment.

I'm experiencing the same issue, too. My environment is as follows.

Grease Pencil's sculpt tools are superior to other products.
I suppose the use of poly lines gives the feature of sculpting (and also animation) an advantage.