Ok, I think I was fooled by something.
Trying to run `emacs -q -l test1.el' with test1.el:
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(setq use-default-font-for-symbols nil)
(set-face-font 'default "Roboto Mono Light 14")
(set-fontset-font "fontset-default" '(#x2600 . #x26ff) "Fira Code 16")
(setq-default mode-line-format '("☰ %b"))
(split-window-vertically)
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I got the following screen:
But it seems that it is not the same font that is used for both modelines.
I would have expected that it was the same.
The font used is switched if I switch windows (C-x o)
So probably I miss something here and there is no bug at all.
Fabrice