The Preference options File Handling > Treat dot/slash As A Keyword Separator doesn't work on import, though it does work with the menu command Metadata > Read Metadata From File.  To reproduce:
 
1. In Preferences > File Handling, set the options Treat dot/slash As A Keyword Separator.
 
2. Import the two attached files slash.jpg and dot.jpg. They contain the following metadata fields:
 
$ exiftool -a -G -xmp:subject -iptc:keywords -xmp:hierarchicalsubject dot.jpg slash.jpg 
======== dot.jpg
[XMP]  Subject  : A.B.C
[IPTC] Keywords : A.B.C
======== slash.jpg
[XMP]  Subject  : A/B/C
[IPTC] Keywords : A/B/C
 
 
3. Observe the following incorrect keywords that were created and added to the imported photos:

 
4. Select the two photos and do Metadata > Read Metadata From File.
 
5. Observe the photos now have the correct hierarchical keyword A > B > C applied:

 
Tested on LR 11.3.1 / Mac OS 11.6.2.