Good evening, @JohnathanVargas ...
In my former role as a Canvas administrator, I recall running into a similar situation. We were test piloting a few of our courses and the Blueprint feature in Canvas. Our Blueprint courses were located in a "Master Term" of Canvas, and the "child" course shells associated with the Blueprint had been automatically created from our SIS (Student Information System). Those course shells also were populated with the course start/end date in course "Settings". After we associated these courses with the Blueprint course and then synced the content, the course start/end dates in those courses were wiped out because the "Master Term" Blueprint course did not have start/end dates in them. So, I suspect something similar is happening with your situation as well.
I could be wrong on this, but although you have the due dates as unlocked, I am not sure that equates to whether or not the due dates will remain after the sync. Locking the due dates means that after the sync occurs, the instructor would not be able to change the due dates at all in his/her course. If the due dates were unlocked, the instructor would be able to change the due dates. But, if there were no due dates in the Blueprint course, I could see why it appeared that the due dates were erased in the "child" associated course(s) after the sync.
I'm not sure if this will help or not, but I thought I'd share my $0.02 ... for what it's worth. Let Community members know if you have any other questions about this...thanks!