Looking for Anne Pedersdatter from Ringebu

My maternal grandmother disappears from the records that I can find after May of 1867 - somewhere between Quebec and Grantsburg, Wisconsin. I like to find some leads on what happened. Anne was born 18 March 1835, Ringebu, Oppland, Norway. 7 May 1861, she married the widower, John Engebretsen Presttangen (b. 20 August 1818) and had one child, Mathias (b. 30 December 1865). In March 1867, Ringebu parish records show John, Anne and Mathias emigrating to "Amerika" with John's older son (first marriage), Engebreth Johnsen, his wife Ronnaug, and their daughter Berthe. Their names are found on the passenger list for the bark "Johannes Rod", sailing 26 April 1867 from Christiana, Norway and arriving 30 May in Quebec. They all (with the exception of Anne) appear in the 1870, 1880 and 1900 US Census returns for Grantsburg, Burnett County, Wisconsin. However by 1880 John is remarried to a Sarah Larson. Any information on how or where Anne Pedersdatter Engebretson disappears in that three year period (1867-1870) would be a help.