Unexpected history expanded in heredoc in $() or <()

[ladyrick]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  
-Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 
-fdebug-prefix-map=/build/bash-7fckc0/bash-4.4=. -fstack-protector-strong 
-Wformat -Werror=format-security -Wall -no-pie -Wno-parentheses 
-Wno-format-security
uname output: Linux n227-006-177 4.14.81.bm.14-amd64 #1 SMP Debian 
4.14.81.bm.14 Sun Aug 25 08:22:56 UTC 2019 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu


Bash Version: 4.4
Patch Level: 12
Release Status: release


Description:
        A heredoc starts with "cat <<'EOF'" is expected to not expand anything 
just like in a single quote string. But when this heredoc is in a $() or <(), 
history is expanded.


Repeat-By:
        This works:
```
cat <<'EOF'
!!
EOF
```
        This doesn't work:
```
cat <(cat <<'EOF'
!!
EOF
)
```
        This doesn't work neither:
```
echo "$(cat <<'EOF'
!!
EOF
)"
```
        Only when I exec `set +H` to disable history expand, these examples all 
work.