SUMMARY
The latest nightly build appimage krita-4.2.0-beta-45bdec0-x86_64.appimage does not display layer content when a file is opened.
The presentation of this bug is slightly different for vector layer content compared to paint layer content but it seems to be influenced by the 'content bounding box'
Turning the layers on then off clears the problem.

STEPS TO REPRODUCE
1. Create an image with a paint layer over the standard background layer and paint a line in it. Save and Close.
2. Open the file and note that the canvas shows all transparency.
3. Turn off then on the background layer and the paint layer and everything displays fine after that.
4. Note that tuning off the paint layer first compared to turning off the background layer first shows slightly different things - a minor detail but naybe useful for diagnosis.
5. Note that using a vector layer (e.g with text) shows a different initial bug characteristic but again related to the content bounding box.

OBSERVED RESULT
As above.

EXPECTED RESULT
This should not happen.

SOFTWARE/OS VERSIONS
Krita

 Version: 4.2.0-beta (git 45bdec0)
 Languages: en_GB, en
 Hidpi: true

Qt

  Version (compiled): 5.12.2
  Version (loaded): 5.12.2

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.9.0-9-amd64
  Pretty Productname: Debian GNU/Linux 9 (stretch)
  Product Type: debian
  Product Version: 9

ADDITIONAL INFORMATION
This does not happen in recent previous nightly appimages.