SUMMARY
When using an icons only task manager and positioning it in the middle of the panel by using spacers with a flexible size to get a ChromeOS style look, opening an application does not resize the icons only task manager and instead ends up with all the apps grouped together in a tiny space when there is much more available to them. Moreso, if you disable flexible size/shrink the panels to give the task manager more room, then expand them again it looks normal.

Before shrink/expand spacers:
https://imagebin.ca/v/5zhjgC2NQifb

After shrink/expand spacers:
https://imagebin.ca/v/5zhjlV7CZFvH

STEPS TO REPRODUCE
1. make a panel with an icons only task manager in the middle
2. remove all the shortcuts there to shrink it as small as possible (makes it easier to see the bug)
3. add a spacer each side and set flexible size.
4. open a few applications

OBSERVED RESULT
The app icons bunch up in a tiny area, and leave blank space around them.

EXPECTED RESULT
The icons would fill the space and push back the spacers

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Manjaro Linux
(available in About System)
KDE Plasma Version: 5.21.80
KDE Frameworks Version: 5.82.0
Qt Version: 5.15.2
Display Server: X11