A wild can complete a set of sevens while the pure run stays natural

Two kings plus a wild joker can be a legal set. Say the wild rank this deal is the nine, so every nine is wild, and the printed jokers are wild as well. You hold the king of hearts, the king of clubs, and a nine. Used as a king, that nine finishes a set of three. The meld is a set, not a sequence. It does not need the three kings to be natural. One wild inside the set is a normal use of the card. Declaring is still illegal if the rest of the hand has no pure sequence, but the failure is not the set. The set did its job.

The set itself can be legal

Put a pure sequence next to that set and the shape becomes clear. Spade 2, spade 3, and spade 4 use no wild. They are the pure sequence. The king-king-nine set sits beside them as a separate meld. The nine was allowed in the set. It is not allowed to replace the spade 3. If you move the nine into the spade run and leave only two natural kings, you no longer have a pure sequence and you no longer have a set of three. Both melds break. The useful picture is the first one: wild inside the set, natural cards only inside the pure run.

The pure sequence beside it cannot borrow that wild

Check the set on its own before you argue about the rest. Three cards, same rank once the wild is assigned, is a set. Then point at the pure run and confirm no wild sits in it. A hand can contain a perfect set and still owe a full count, on a common sheet often 80, because the pure sequence was never natural. Houses differ on the points. They do not differ on this split. Build the set with the joker if that is the card you have. Keep one natural run untouched. Show only when both are on the table.