It does not make sense, no. However lets see if we can track this.It sounds like we have a workbook that's a .twbx and it has several sheets. Are you editing your current connection or adding a new connection. These are different one will replace all instances of the original data source and the other adds a new data source in so you will have two data sources.
It's possible that this is making sheets hide inside of a already built dashboard (but I doubt it). Can you duplicate this with a before or after so myself or someone with more time can dissect it and report back some possible causes?
Try these steps and check
If the model is currently using a non-server data source, we'd first publish it to the server
1) Create local copy of server data source
2) Replace server data source with local data source in workbook
3) Remove reference to server data source (close connection)
5) Publish data source to server
6) Replace local data source with server version
7) Remove (close) local data source
9) Republish workbook to server
we don't have server...
always when connecting to a new data source (user would send me their tableau file - twb in this example - and data source -accdb) and I would save to me PC and Edit Connection (it would remember theirs)...I can connect the tableau file to the data source on my PC, but when I go to review the tabs (user questions), I would find the missing tabs (always all of them, but one)...?