I think it is a bug for the dialog pop up after the windows system is locked.
Winform, .Net 4.8, C#
Say there are a main form, form A and form B.
Open the main form, click a button on it to pop up form A (with modal or modeless, the owner is the main form), and start a timer which will pop up form B after 5 seconds
Press Win + L to lock windows
The timer is triggered in the background to pop up form B (with modal, the owner is the main form)
Input password to unlock windows
The form B will turn to back of form A, that the form A at top most, but the keyboard and mouse does not work on it.