Here's an output from devel branch bash and 5.2.26:
$ bash-99999 -c 'echo "$BASH_VERSION"; x="abc#xyz"; echo "${x/\#}"'
5.3.0(1)-alpha
abc#xyz
$ bash-5.2.26 -c 'echo "$BASH_VERSION"; x="abc#xyz"; echo "${x/\#}"'
5.2.26(1)-release
abcxyz-- konsolebox
