SUMMARY
When Dolphin sorts folders by size (instead of item counts) Dolphin switches to sorting folders first even when sorting folders first is not requested.

STEPS TO REPRODUCE
1. Launch Dolphin normally and go to a folder where you have many files and folders of varying sizes.
2. Open Dolphin preferences and switch View Modes/Details/Folder size displays to "Size of contents, up to" and pick some depth level that will certainly contain everything in your folder and all subfolders.
3. Be sure that View->Sort By->Folders First is disabled (unchecked).
4. Switch a Dolphin window to details mode and add the Size column if you're not seeing sizes.
5. Click the Size column to sort the table by size.

OBSERVED RESULT
Dolphin sorts folders first despite being told not to do this in step 3.

EXPECTED RESULT
Dolphin should only sort folders first when View->Sort By->Folders First is enabled.

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2
Kernel Version: 5.10.0-3-amd64
OS Type: 64-bit
Processors: 8 × AMD FX(tm)-8150 Eight-Core Processor
Memory: 31.3 GiB of RAM
Graphics Processor: NV106

ADDITIONAL INFORMATION
I'm guessing that this (now erroneous) behavior is a holdover from when Dolphin did not sort by actual file/folder sizes but by item counts in folders? Now that Dolphin can sort by file sizes (yay for that feature!) the sort order should not implicitly sort folders first unless requested.