In "Scanning faces" dialog with all previous faces removed selecting "Detect and recognize faces" works same as just "recognize faces" (progress bar rushes to 100% without any significant work).
Only when selecting just "detect faces" it really does detection.

Expected: When selecting "detect and recognize faces" digikam should first run detection and recognition.

Reproducible: Always

Steps to Reproduce:
This is with all old face tags removed, otherwise it's not so obvious (I believe)
1. From people tags click "scan collection for faces"
2. Select "detect and recognize faces" and some albums
3. click scan
Actual Results:  
No real work done, no faces detected (takes few seconds for thousands of photos)

Expected Results:  
Digikam spends few hours looking for faces

I have debug output enabled which confirms what happens in gui. Log entries suggesting face detection only appear when run just "detect faces"

Running current git. 

Workaround: run "detect faces" and then "recognize faces" instead of the one-step option.