grep won't handle sed output as a newline delimited list

Question

Playing around today, I was trying to get grep to consume the output of...

$(apt-cache rdepends hunspell-fr | sed -e 's/^\s*|\?//' -e 1s/^/\'/g -e \$s/$/\'/)

...as a newline delimited list of patterns nested within single quotes. When I run the above command and then simply copy/paste the resulting output into a new grep command, it behaves as expected...

nohatsatthetable@debian:~$ dpkg -l | grep 'hunspell-fr
Reverse Depends:
hunspell-fr-classical
thunderbird-l10n-fr
firefox-esr-l10n-fr
thunderbird-l10n-fr
task-french-desktop
hunspell-fr-revised
hunspell-fr-revised
hunspell-fr-comprehensive
hunspell-fr-comprehensive
hunspell-fr-classical
firefox-esr-l10n-fr'

ii  firefox-esr-l10n-fr                    102.4.0esr-1~deb11u1             all          French language package for Firefox ESR
ii  hunspell-fr                            1:7.0-1                          all          French dictionary for hunspell (dependency package)
ii  hunspell-fr-classical                  1:7.0-1                          all          French dictionary for hunspell (classical version)
ii  task-french-desktop                    3.68+deb11u1                     all          French desktop

However when I run the following one-liner, which to my eye should be functionally the same, grep only interprets the first line as a pattern, interpreting all subsequent lines (or words in the case of the second line) as the name of a directory or file to search within for the pattern...

nohatsatthetable@debian:~$ dpkg -l | grep $(apt-cache rdepends hunspell-fr | sed -e 's/^\s*|\?//' -e 1s/^/\'/g -e \$s/$/\'/)
grep: Reverse: No such file or directory
grep: Depends:: No such file or directory
grep: hunspell-fr-classical: No such file or directory
grep: thunderbird-l10n-fr: No such file or directory
grep: firefox-esr-l10n-fr: No such file or directory
grep: thunderbird-l10n-fr: No such file or directory
grep: task-french-desktop: No such file or directory
grep: hunspell-fr-revised: No such file or directory
grep: hunspell-fr-revised: No such file or directory
grep: hunspell-fr-comprehensive: No such file or directory
grep: hunspell-fr-comprehensive: No such file or directory
grep: hunspell-fr-classical: No such file or directory
grep: firefox-esr-l10n-fr': No such file or directory

Why does it behave in this way?

Answer 1

Yes, as mentioned by @Kusalananda , the quotes are mandatory:

$ dpkg -l |
    grep "$(apt-cache rdepends hunspell-fr |
        sed -e 's/^\s*|\?//' -e 1s/^/\'/g -e \$s/$/\'/
 )"