Batch ReplaceValue Text does not deliver 'null'

Since this is a single value list, hence Text.Combine is not needed. This is conveting nulls into blanks. Use below formula

= Table.ReplaceValue(Quelle,
each [Responsible],
each List.ReplaceMatchingItems({[Responsible]},{{"#", null}, {"n.n.", null}, {"na", null}}, Comparer.OrdinalIgnoreCase){0},
Replacer.ReplaceValue,{"Responsible"})

You could optimise it a little bit by skipping all the extra info provided and just customise the Replacer funcion:

Table.ReplaceValue(
    Quelle,
    {"#", "n.n.", "na"},
    null,
    (x, y, z) as nullable text =>
        if List.MatchesAny( y, each _ = Text.Lower(x) )
        then z else x,
    {"Responsible"}
)

That way, you do not feed it [Responsible] 3 times per calculation, and the check stops once it finds a match, instead of trying to replace every value and if the value is replaced then it replaces the actual value in the record (row).

Cheers,