When the beat grouping default in a 7/8 measure is set to e.g. 3+4 or 3+2+2, the notes are beamed as 3+3+1. Other combinations of grouping settings and time signatures may also lead to incorrect results.
Solution
If the default beat grouping does not lead to the desired result, the notes can be beamed manually. Select the notes to be beamed and use the local menu function Attributes > Beaming> Beam selected (or the corresponding keyboard shortcut).