>> So maybe fido-mode could be fixed to always provide the default value
>> as the first highlighted candidate?  Then RET will select it.
>
> Another part of the problem is still unsolved and the question still is:
> shouldn't fido-mode bubble the default value to the top of the list
> even when the default value doesn't exist in the list of completion
> candidates?  This is the same question as was asked in
> https://debbugs.gnu.org/38992#76

It seems there is no way to change the behaviour of fido-mode,
so what remains to do is to add the default values to the completion list
only in rgrep, then fido-mode will highlight the default at the top: