SUMMARY
If we have an existing static notification, and there are multiple active/ongoing notifications (say, 2 ongoing dolphin file transfer notifications) and one of them is cancelled, then the other notification suffers from a glitch where their size can change and their progress bar appearing below the static notification.

STEPS TO REPRODUCE

METHOD 1:
1. Have some static notification from any app show up in the notification popup
2. Start two dolphin file transfers to different directories so it shows two active notifications
3. Open notification popup and cancel one of the file transfers
4. The 2nd ongoing dolphin notification glitches, contracting in size horizontally and its progress bar shifts below the static notification

VIDEO: https://imgur.com/a/EMujbP4

METHOD 2:
1. Open and pin the notification popup
2. Have some static notification in the popup
3. Now start dolphin file transfers, each one adding a new notification
4. Inside the popup, the notifications from dolphin will start overflowing over the static notification.

VIDEO: https://imgur.com/a/z9nuy5x

OBSERVED RESULT
Glitches when notifications are added/updated

EXPECTED RESULT
No glitches

SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 40
KDE Plasma Version: 6.2.2
KDE Frameworks Version: 6.7.0
Qt Version: 6.7.2
Kernel Version: 6.11.4-201.fc40.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 4 × Intel® Core™ i5-6300U CPU @ 2.40GHz
Memory: 11.1 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics 520
Manufacturer: HP
Product Name: HP ProBook 640 G2

ADDITIONAL INFORMATION
I have experienced this in Plasma 6.1 as well, probably on 6.0 and maybe on 5.x versions too, had forgotten to report :-)