SUMMARY
In verion 4.3.0, the horizontal Move shortcut keys work to move the content of a frame on an animated layer.
From version 4.4.0 onwards to the Apr 21 5.0.0-prealpha (git 3eda0b9) they do not work on a frame of an animated layer.

STEPS TO REPRODUCE
1. Create a simple animated layer with content (only one frame is needed).
2. Select the Move tool and use the vertical Move shortcut keys (up/down cursor arrows)
3. Use the horizontal Move shortcut keys (left/right cursor arrows).

OBSERVED RESULT
2. The content bounding box (and its image content) move up and down.
3. The content bounding box is removed and there is no movement.

EXPECTED RESULT
3. The movement should happen as it does for the vertical shortcut keys.

SOFTWARE/OS VERSIONS
Krita

 Version: 5.0.0-prealpha (git 3eda0b9)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: false

Qt

  Version (compiled): 5.12.9
  Version (loaded): 5.12.9

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-16-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10
  Desktop: MATE