> In Emacs -Q there is not this issue.

This indicates the issue is with your config.

I suspect it can be from theme.

When I make a search for "theme" word in my init.el file, I get just one result:

 '(custom-safe-themes '(default))

Are you sure there is no config editing org-level-* faces somewhere?

When I make a search for "org-level", I only get org-level as inheritance (aside from the set-face-attribute I shared before). Example:

 '(org-upcoming-deadline ((t (:inherit org-level-3))))

> (set-face-attribute 'org-level-1 nil :inherit 'org-default :height 1.5)
> (set-face-attribute 'org-level-2 nil :inherit 'org-level-1 :height 1.4)
> (set-face-attribute 'org-level-3 nil :inherit 'org-level-1 :height 1.2)

Also do note height work as an increment from inherited value. It does
override. So according to this config org-level-2 height will be 1.4x
times that of org-level-1.

> When I evaluate (describe-face) on any headline level, it consistently
> reports that it is org-level-1.
I assume you are calling it interactively (via M-x), this gets
completing-read and allows you to select. Maybe you got only org-level-1
and selectec it?

I call (describe-face) interactively, and it offers a default selection. 

This default selection is always the face where the point is on. Although the point was on a headline of level 2, 3, etc, when calling (describe-face) the default selected face is always org-level-1.

Try this,

(set-face-attribute 'org-level-1 nil :height 1.5 :foreground "#b4befe")
(set-face-attribute 'org-level-2 nil :height 1.4 :foreground "#f5deb5")

org-level-1 changes are applied to every org-level:

https://ibb.co/XW2KSF3

Again, if it works in emacs -Q or vanilla org-mode. You need to debug
your init and theme.

I know. I tried bug hunter. 

Any help?

--
Best,
Dilip