There is one defect it sounds like you may be running into: CLRT-80172 that is fixed in 15.1. Below are the details, if this doesn't seem to be the one you are facing, let us know.
Steps to Reproduce:
1. Create an Idea with the following dates: 4/4/16 - 4/8/16
2. Add a resource to the team
3. On the Staff Member Properties page of the resource added on Step2, set the Default Allocation % to 0% (reproducible with any other value as well)
4. Add the following Planned Allocation Segment:
4/6-4/8 : 50% (4hours x3 days = 12 allocation hours)
5. Notice that a default allocation segment will get created. This segment will be inheriting the Default Allocation % (0%) for the following dates, and will be read only:
4/4-4/5: 0% (This is a 2 day segment.)
6. Submit the Idea for Approval, Approve the Idea
7. Click on the Convert button on the Idea properties page to convert the Idea to a Project
8. On the Select Investment Type page, select the 'Copy Team' checkbox.
9. On the Create Project page, leave the start and Finish dates as they are. (They will be the same as the dates on the Idea by default.)
10. Click on Save
11. Go to the Team detail tab
Expected Results: The default allocation segment to be the same as they were on the Idea. Allocation to be 4 hours a day for 4/6, 4/7 and 4/8.
Actual Results: The duration of the 0% allocation segment has changed from 2 days to 4 days on the Project. The Project Allocation segments are as follows: 4/4-4/7 at 0% (this is now 4 days) and only the remaining day, 4/8 has an allocation of 50% (4 hours total).
Workaround: Manually create a segment with the same allocation % as the Default Allocation % for that team member on the Idea before converting it.