This might be WAI as I don't see any articles or documents that can confirm expected result as 'remain fullscreen' after alt-tabbing or locking the screen. 
This is also different from 'maximize' behavior that most apps do support and remain maximized after locking.
As I'm not completely sure this is WAI, let's check the expected behavior with engineers.

I am the original reporter of this issue. The big problem it causes for us is with Citrix Receiver. If someone has Receiver (aka Xendesktop) open across multiple monitors and has windows open on each monitor, then locks and unlocks their computer, it loses fullscreen and shoves all the windows into 1 monitor.

Then the user has to go back into fullscreen to get multi-monitor back and re-organize their windows again. This happens every time that they lock which is probably at least a few dozen every day. This particular issue is the only thing preventing our organization from going full Chrome-OS for our thin client needs.

warx@ please work with ovanieva@ to determine how to handle this issue.

Hello!
This bug is receiving this notice because there has been no acknowledgment of its existence in quite a bit of time
- If you are currently working on this bug, please provide an update.
- If you are currently affected by this bug, please update with your current symptoms and relevant logs.

If there has been no updates provided by EOD Wednesday, 12/19/18 (5pm EST), this bug will be archived and can be re-opened at any time deemed necessary.

Thank you!

Due to lack of action this bug has been Archived. If work is still being done on this issue or you are still experiencing this issue please feel free to re-open with the appropriate information.