SUMMARY
If you have a Floating panel + Fit Content, it might render incorrectly at start up: it will show as a small panel and you are unable to see all the panel's content. This happens consistently at every sessions start, and this problem happens across Plasma versions, from 6.0 to the current's version. 

The weird part is, this doesn't happen if you have a fresh Plasma installation, so I suspect that this is a caching problem. but in my case, updating the cache doesn't help. It's always happen after you've used Plasma for a while in every platform I tested.

I have tested this on two different laptops, and the main one (which I include in the SOFTWARE/OS VERSION section) also tested with Fedora as well. It shows the same result's. Note that I reinstalled Plasma on 6.2
 on each ones, and this might be a regression(?) of https://bugs.kde.org/show_bug.cgi?id=477317. 

My suspicion is that the panel's frame doesn't update properly until you triggered any actions that updates the panel's state.

STEPS TO REPRODUCE
1. Have a floating panel. Turn off all "updating" components.
2. Log in, you will see the incorrectly
3. Do any action that can trigger the panel's frame update, like launching an app. It will show correctly

OBSERVED RESULT
Floating panel + Fit Content setting doesn't render correctly

EXPECTED RESULT
Floating panel + Fit Content setting rendered correctly at start.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.2.4
KDE Frameworks Version: 6.8.0
Qt Version: 6.8.1
Kernel Version: 6.12.4-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 2 × Intel® Pentium® 3558U @ 1.70GHz
Memory: 7.7 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics

ADDITIONAL INFORMATION
there's a similar bug's report on TumbleWeed with no reply https://forums.opensuse.org/t/kde-plasma-panel-not-displayed-properly-when-first-logging-in/180422.