If the Meridian flip warning option has be enabled, Mosaic will issue a warning if it thinks the next slew is on the opposite side of the meridian from last slew position. Mosaic itself doesn't make the mount flip, the driver actually controls the flip. The mount may not actually flip when you click the [OK] button because, in the time it takes you to make the decision, the target position may have drifted back to the mounts current side.
When doing a mosaic of an item that starts spanning the meridian, a recommendation would be to start on the western side and move through the grid by doing columns of common RA. In this way you spend the maximum amount of time at the same RA. If the second column is on the other side of the meridian when you start, then by by the time you've finished imaging the the first column, the second column may well have drifted across the meridian and so no flip would be needed.
If you do get the meridian flip warning in mosaic you can always cancel the slew, wait a short until the object moves onto the current side of the meridian.