SUMMARY
Double-click dragging over text should consistently select words. But one can see that this is inconsistent depending on the rename-inline setting.

STEPS TO REPRODUCE
1. Set Dolphin preferences (General page) to turn on renaming inline.
2. Find or make a file with a filename that has words in it (I'll use "foo bar baz.txt" as an example).
3. Rename that file and double-click on a word (say, "foo").
4. On the second click, drag across the other words. Notice that "bar" and "baz" are selected as words as you drag.
5. Cancel the rename (press escape) and switch the preference to disallow renaming inline.
6. Rename the same file as before and again double-click on a word ("foo").
7. On the second click, drag across the other words.

OBSERVED RESULT
Notice that "bar" and "baz" are now selected as characters as you drag, not words.

EXPECTED RESULT
I expected that "bar" and "baz" are selected as words.

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux 
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.14.2
Kernel Version: 5.7.0-1-amd64
OS Type: 64-bit
Processors: 8 × AMD FX(tm)-8150 Eight-Core Processor
Memory: 31.3 GiB of RAM

ADDITIONAL INFORMATION
I can reproduce this same effect of double-click dragging selecting characters instead of words in other applications that use KDE or Qt such as Qbittorrent. I don't know enough about the hierarchy of code involved to know if that means this is a KDE problem, a Qt problem, or a bug that is app-specific but happens to manifest in apps I've tried.

Thanks.