The initials some recount observers saw on ballots are municipal clerk’s employees, not voters. The clerk or deputy clerk is required to initial the absentee ballot before issuing it to the voter, so it is natural that many of them all have the same set of initials.
Unfortunately, recount observers did not understand what these initials were for, and they assumed it meant the same voter filled out and initialed all the ballots. Here are links to the state laws that require initials on ballots:
https://docs.legis.wisconsin.gov/statutes/statutes/5/ii/54
5.54 Notice to electors. Every ballot, except a voting machine ballot, shall bear substantially the following information on the face: “NOTICE TO ELECTORS: This ballot may be invalid unless initialed by 2 election inspectors. If cast as an absentee ballot, the ballot must bear the initials of the municipal clerk or deputy clerk."
https://docs.legis.wisconsin.gov/statutes/statutes/6/iv/87
6.87 Absent voting procedure.
(1) Upon proper request made within the period prescribed in s. 6.86, the municipal clerk or a deputy clerk authorized by the municipal clerk shall write on the official ballot, in the space for official endorsement, the clerk's initials and official title.
It is election fraud for a voter to sign or initial their own ballot. Here is the law about election fraud:
https://docs.legis.wisconsin.gov/statutes/statutes/12/13
12.13 Election fraud.
(1) ELECTORS. Whoever intentionally does any of the following violates this chapter:
(f) Shows his or her marked ballot to any person or places a mark upon the ballot so it is identifiable as his or her ballot.