SUMMARY
digiKam does not store Dimensions information when saving Face Tags in xmp:mwg-rs.
Why is this important? Because Lightroom can't import the digiKam photo collection properly. It does not recognize all Face Tags and treats them as "Unnamed people".
This may prevent many people from using digiKam, fearing that they will lose the entire collection of Faces tags if they switch to Lightroom in the future.

STEPS TO REPRODUCE
1. Recognize any person in the photo
2. Import this photo into Lightroom
3. Try to see named Face Tag of this person in the imported photo

OBSERVED RESULT
You can see only "Unnamed people"

EXPECTED RESULT
All people recognized properly as "Person's name"

ADDITIONAL INFORMATION
I compared the xmp tags between the photos that were imported correctly and the digiKam photos and found only one important difference, it is Dimensions block inside of mwg-rs section. It contained height and width size of the picture and format (pixel), that's all.
If you export xmp tags with "exiftool -tagsfromfile SRC.EXT -xmp:all DST.xmp" That block looks like this:

 <rdf:Description rdf:about=''
  xmlns:mwg-rs='http://www.metadataworkinggroup.com/schemas/regions/'
  xmlns:stArea='http://ns.adobe.com/xmp/sType/Area#'
  xmlns:stDim='http://ns.adobe.com/xap/1.0/sType/Dimensions#'>
  <mwg-rs:Regions rdf:parseType='Resource'>
   <mwg-rs:AppliedToDimensions rdf:parseType='Resource'>
    <stDim:h>1920</stDim:h>
    <stDim:unit>pixel</stDim:unit>
    <stDim:w>2560</stDim:w>
   </mwg-rs:AppliedToDimensions>
...

digiKam never write this  <mwg-rs:AppliedToDimensions> block and that is the only reason that prevents the correct import Face tagged photos into Lightroom.