Regexp string in := parameter expansion triggers failglob

[polynomial-c]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: x86_64-pc-linux-gnu-gcc
Compilation CFLAGS: -march=native -mtune=native -O2 -pipe -Wno-parentheses 
-Wno-format-security
uname output: Linux abudhabi 4.19.13 #1 SMP PREEMPT Sat Dec 29 15:21:48 CET 
2018 x86_64 AMD Opteron(tm) Processor 6328 AuthenticAMD GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 0
Release Status: release

Description:
        When sourcing a script that contains a variable which performs
        a := parameter expansion with a regular expression while failglob
        shell option is enabled, bash-5.0 emits a "no match: <regexp>" 
        error message.

Repeat-By:
        > cat bash5_testscript.sh 
        : ${TESTPATTERN:="\(auto|unit\)\?tests\?"}
        echo ${TESTPATTERN}

        > shopt -s failglob
        > source bash5_testscript.sh
        bash: no match: \(auto|unit\)\?tests\?
        bash: no match: \(auto|unit\)\?tests\?
        >