unsetting associative array executes commands

[Jason A. Donenfeld]

This behavior is quite surprising:

$ declare -A blah
$ blah['$(DOESNOTEXIST)']=broken
$ for i in "${!blah[@]}"; do echo "$i"; done
$(DOESNOTEXIST)
$ for i in "${!blah[@]}"; do unset blah["$i"]; done
bash: DOESNOTEXIST: command not found
bash: unset: [$(DOESNOTEXIST)]: bad array subscript

I wouldn't have anticipated the double expansion of "$i" there.

Is this a bug? Bash 5.1p4.