Unexpected result of array assignment

[Darren 'Tadgy' Austin]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -fdebug-prefix-map=/build/bash-LQgi2O/bash-5.0=. 
-fstack-protector-strong -Wformat -Werror=format-security -Wall 
-Wno-parentheses -Wno-format-security
uname output: Linux phil 5.0.0-13-generic #14-Ubuntu SMP Mon Apr 15 14:59:14 
UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 3
Release Status: release

Description:
        There seems to be unexpect (a bug?) behaviour with bash's handling of 
array assignments.
        Specifically, if you try to assign an array element a value containing 
that same array value, it fails to work.
        See example below :)
        Bug confirmed by the folks in #bash on Freenode - unable to explain 
what is happening.

Repeat-By:
        declare -A foo
        foo=(["key"]="value1")
        declare -p foo
        foo=(["key"]="${foo["key"]} value2")
        declare -p foo

        The above should result in 'foo["key"]' having a value of 'value1 
value2', but the result is simply ' value2', which I believe to be incorrect 
behaviour.