SUMMARY
When using the back button on the mouse to navigate back, double-clicking this button to quickly navigate back, the second click is instead treated the same as a regular left mouse button double-click, triggering an open action.  As such: first click navigates one back, second click "activates" whatever is under the cursor.  So when over a folder, descends into the folder, when over a file, opens the file.

STEPS TO REPRODUCE
1. Open dolphin, go to e.g. home directory.
2. Create an empty directory, let's call it "A".
3. Enter directory "A".
4. Create an empty directory, let's call it "B", in "A".
5. Hover the cursor over "B".
6. Enter directory "B" (double click), do *not* move the cursor.
7. Double click the mouse back button.

OBSERVED RESULT
Back in directory "B".

EXPECTED RESULT
In directory containing "A".

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Manjaro Stable (Kernel 6.9.5-1-MANJARO 64-bit)
KDE Plasma Version: 6.0.5
KDE Frameworks Version: 6.3.0
Qt Version: 6.7.1

ADDITIONAL INFORMATION
To test with files, follow the repro steps but create an additional file in "A", hover that file, enter "B" (e.g. using keyboard, or remember where to place the cursor so it's over the file), then do the double-back-click.