can't unset hash item with specific key

[l . bashbug]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -fstack-protector-strong -Wformat 
-Werror=format-security -Wall 
uname output: Linux h2 5.15.0-2-amd64 #1 SMP Debian 5.15.5-1 (2021-11-26) 
x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1
Patch Level: 12
Release Status: release

Description:
        making left square bracket char ([) part of the key into a hash causes
        unset to silently fail when trying to unset that specific hash item.
        The position of left square bracket char in the string used as key 
doesn't
        matter. unset also fails when the key is a single [ character.
        Please refer to the following script which attempts to demonstrate this 
quirk.

Repeat-By:

# diagnostic shows remains of hash after attempts to unset single item.
show()  { echo "$1, ${#a[@]} items in a: a['${!a[@]}']=${a['[foo]']}"; }


# prepare test scenario

declare -A a                            # make "a" a hash
a['[foo]']="bar"                        # assign "bar" to item keyed [foo]
show "good"                             # correctly output item, show correct 
item count


# intention is to get rid of the item again, by unsetting the single item, not 
the whole array.
# several ways of quoting are attempted here

unset "a['[foo]']"                      # try to remove item
show "wrong"                            # can still output item, item still 
counts

unset 'a["[foo]"]'                      # try to remove item
show "wrong"                            # can still output item, item still 
counts

unset 'a[[foo]]'                        # try to remove item
show "wrong"                            # can still output item, item still 
counts

unset "a[[foo]]"                        # try to remove item
show "wrong"                            # can still output item, item still 
counts

unset "a[\[foo]]"                       # try to remove item
show "wrong"                            # can still output item, item still 
counts

unset "a["\[foo]"]"                     # try to remove item
show "wrong"                            # can still output item, item still 
counts

unset "a['['foo]]"                      # try to remove item
show "wrong"                            # can still output item, item still 
counts

a["[foo]"]=""                           # Best I can currently do
show "set empty, item still counted"    # item still there though contents are 
empty string.