SUMMARY
If you make a vector selection, then another one and then undo the second one (so the first one appears), the selection is broken: the outline is correct, but the global selection mask and the reality (aka fill tool) shows that only the part of the image that was selected by *both* selections (that was in the intersection on both the undone and the previous selection) is actually selected now.

Tool options: vector, 'replace'.

STEPS TO REPRODUCE
1. Create one rectangular selection.
2. Create another one that intersects with the previous one.
3. Undo.
4. Use Fill Tool to fill the selection.

OBSERVED RESULT
Fill Tool only fills the intersection of selections from 1. and 2.

EXPECTED RESULT
Fill Tool fills the entire selection.

ADDITIONAL INFORMATION
It works for pixel selection.



SOFTWARE/OS VERSIONS
Krita

 Version: 4.3.0-prealpha (git 47a57162)
 Languages: en_US
 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: 4.15.0-45-generic
  Pretty Productname: Linux Mint 19.1
  Product Type: linuxmint
  Product Version: 19.1


Hardware Information

  GPU Acceleration: none
  Memory: 7771 Mb
  Number of Cores: 4
  Swap Location: /tmp