The short answer: That is correct, you cannot change the start if there are actuals the same way you could when there were no actuals.
Say you create a project with an effort task with start date March 1 and add a resource to the team.
No problem setting the start date of the effort task one month later (next working day)
If you post time say at the beginning of May
That will change the start of the effort task to the date of first posted actual ie May 1 in this case
As you can see it is read only.
If you put some actuals before the start of the Effort task you can move the start earlier
but it is still read only