SUMMARY
The quick-tiling-windows behave a lot like 'actual'-tiling-windows. After manually resizing the new Quick tiling windows (i.e. with META+arrow keys), and then resizing the windows, causes windows to connect with each-other and size-changes to persist. 

STEPS TO REPRODUCE
1. open at least 1 window
2. use default kwin shortcut to quick tile to vertical half: e.g. META+left 
3. Drag to resize the window
4. Use quick tiling shortcuts again

OBSERVED RESULT
When I use META+arrow again after resizing, the adjusted window will quick-tile into the manually adjusted size, and not to the expected 1/2 of the monitor. Size and position persists when quick-tiling shortcuts are repeated, also for newly opened windows. 
Windows also connect with other windows when multiple windows are open.

EXPECTED RESULT
The expected result is that Quick-tiling shortcuts would always 're-snap' windows to the fixed, designated position, such as the left-half of the monitor with META+left, regardless of any manual resizing that happened since the last quick-tiling action. Expected is that windows still behave like independent floating windows after being quick-tiled to a certain position.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon Testing Edition x86_64
KDE Plasma Version: 5.26.90
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION
I also tested the new tiling feature in the 5.27 beta, which has similar features to the unexpected behavior. I am reporting here specifically regarding quick-tiling, and with the (possibly wrong) assumption that quick-tiling is supposed to still function as before, independently from the new tiling capabilities.