Hi!
While I don't work for instructure, I might be able to point out a few things that may be helpful.
First off, ensure that the assignments under the module you locked do not have due dates that would override the module being locked. Whenever you lock a module and schedule it to be unlocked on a specific date, if an assignment has a different due date, it will override the system at times, that's happened to me as well.
Second, make sure you didn't duplicate anything from other modules that would result in due dates changing. For example, in module one if you had a due date for lets say October 1, and you duplicated that assignment and moved it to a later module (lets say module 2), canvas may think you want the same due dates as the assignment you originally duplicated from, resulting in it being unlocked for your students. So make sure you didn't accidentally duplicate due dates.
Third, this kind of relates with the one above. If your assignment has different availability settings under a module that will unlock at a different time, chances are that button will be unlocked and will unlock the entire module (as we can't grey out individual buttons in modules).
There are other things that can cause this but start off here and see if this works. Make sure that all your assignments under the module have the same availability date (thats different than the due date), or better off make don't have an availability date at all, as that can also mess things up.
Sorry if this was confusing, ask questions I will clear stuff up.
Noah