This bug can only be reproduced under Windows 10. A File Layer can only update once after the document is opened. Krita won't update the a File Layer for the second time, even when the source file has been updated. Happened to both: directly saved KRA and exported PNG file layers. Tested on Windows 10 2004. 4.3.0 beta 2 X64. This is a bug I relay from a manga artist. It is causing him trouble because he uses File Layers in attempt to reduce document size.
I have just fixed this bug in https://invent.kde.org/graphics/krita/commit/1b912acc814f616772ef17a1a5020af257d45d51 This commit also fixes the possible loss of connection that could cause this very bug. *** This bug has been marked as a duplicate of bug 411790 ***
Thank you Dmitry!