I’ve tested this on my Mac with Lightroom, Capture One and PhotoLab 4.
They use different tags and tag groups as you can see in the screenshot.
- C1 writes IPTC Keywords, XMP Subject and XMP Hierarchical Subject tags
- Lr writes IPTC Keywords and XMP Subject tags (blue lines above)
- PL writes XMP Subject and XMP Hierarchical Subject tags
All exported files show the keyword “Türen” correctly in Apple Preview.
My guess is, that piwigo reads IPTC only, therefore missing the keyword written by PhotoLab 4.