Hi David,
The shifts can be as locked down or as flexible as you configure them to be. If the shifts across the days have a wide start time, there is nothing to constrain the scheduling engine in picking any valid start time within those windows across the days (as long as they do not violate other constraints such as the minimum time between shifts). The scheduling engine is free to schedule exactly one shift among the choices per day. E.g., choose an early shift / start time on Monday and a late shift / start time on Wednesday.
We do plan on releasing a 'Maximum shift start time variance' work plan constraint. This will attempt to constrain the start times, if there are common ones, across shifts / days.
Until then you could narrow down the shift start window to some acceptable tolerance and if it still is not constrained enough, then utilize fixed shift start times with some manual re-assignment/constraint change on 1-week, 2-week, etc. rotation.
Typically users will setup several work plans (e.g., morning, morning flex, second shift, evenings, etc.). Having one work plan with wide open shift start time possibilities is not very common. Could it be that you have distinct types of shifts that could be better segmented in their own work plans? E.g., instead of having one work plan with three shifts: shift 1 with a start window of 8am-12pm, a shift 2 with a start window of 12pm-4pm, and a shift 3 with a start time of 4pm-12am (next day) - have three work plans (i.e., first shift, second shift, third shift) with one shift each.