Different history expansion behaivor: true `# # !xxx` vs. true `# !xxx`

[Clark Wang]

This is from stackoverflow ( https://stackoverflow.com/questions/60166019/ ) 
and it looks like a bug:

$ bash --version
GNU bash, version 5.0.7(3)-release (x86_64-pc-linux-gnu)
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
$
$ set -H
$ true `# !xxx`
bash: !xxx`: event not found
$ true `# # !xxx`
$