SUMMARY
First, this is not the same as 420555 (I have found that too and saw the bug report for it).

When you have natural sorting enabled, Dolphin will not list the shorter file before a longer one if the longer one have an underscore at the position of the shorter file dot.

For example, I have files:
- file.txt
- file_with_underscore.txt

In natural mode, Dolphin will list them as

- file_with_underscore.txt
- file.txt

In alphabetical mode, it will list them in 'correct' order.

STEPS TO REPRODUCE

- Have sorting set to 'natural mode' (note that you must take care of bug 420555, which require you to re-open the config dialog if you changed the setting)
- Create 2 files in a directory, named "file.txt" and "file_with_underscore.txt"

OBSERVED RESULT

You see:
- file_with_underscore.txt
- file.txt

EXPECTED RESULT

Should discard extension part when comparing file names, as in alphabetical sort mode, or more specifically, as you get when you do ls -l with LC_COLLATE=C.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 20.04
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8