The LabeledScale in tkinter.ttk seems to have some kind of hidden element that covers the LabeledScale's label when the value is set to mid-scale. Tested on Windows 10, Python 3.6

See below code to reproduce:

import tkinter
from tkinter import ttk

master = tkinter.Tk()
_out1Value = tkinter.IntVar(master)
out1Slider = ttk.LabeledScale(master, from_=-100, to=100, variable=_out1Value, compound="bottom")
_out1Value.set(0)

# uncomment to "fix"
# out1Slider.label.lift()

out1Slider.pack()

master.mainloop()

3.6 only get security fixes, but your nice minimal, complete verifiable example let me verify easily on 3.9a5 with tk 8.6.9 and 3.6 with 8.6.6 and 2.7.17 with 8.5.19. The hidden element appears to be something like'||' with only part visible, depending on the scale value.  It at least partly obscures from -20 to 15.  (Clicking the bar moves 1 unit at a time.)  This should be tested on other systems, but I suspect that this is an upstream tcl/tk issue.

This is indeed a tkinter issue and is caused by the placeholder label. As the main label is added to the frame using the place manager, this placeholder is required to make the frame the correct height.

I have attached a small diff which lowers this placeholder, fixing the problem (raising the main label does the same thing).

Right.  LabelScale is a tkinter.ttk extension, like OptionMenu.  If I do the PR, I would call the placeholder 'dummy' since it is permanent (not removed after initialization).  I'd like to see what happens without it and expand the comment to a line like
    # Dummy needed to ... .

New changeset b9ced83cf427ec86802ba4c9a562c6d9cafc72f5 by E-Paine in branch 'master':
bpo-40219: Lowered ttk LabeledScale dummy (GH-21467)
https://github.com/python/cpython/commit/b9ced83cf427ec86802ba4c9a562c6d9cafc72f5

New changeset 13d40c2a418116797eccd77bd65e4dbd689008db by Miss Islington (bot) in branch '3.8':
bpo-40219: Lowered ttk LabeledScale dummy (GH-21467)
https://github.com/python/cpython/commit/13d40c2a418116797eccd77bd65e4dbd689008db

New changeset 6ad5fd14825fc6039a9684dfdc14f5d12b86e25f by Miss Islington (bot) in branch '3.9':
bpo-40219: Lowered ttk LabeledScale dummy (GH-21467) (GH-23788)
https://github.com/python/cpython/commit/6ad5fd14825fc6039a9684dfdc14f5d12b86e25f