Hi Steffen and welcome to the forum,
perhaps you may read the CR3 using exiftool (see https://exiftool.org/ for the Windows x64 download) to see which lens and crop factor is reported. I had a similar issue with a Sigma Art lens, that was included in the database, but not recognized for my Nikon F mount. I had to provide an exiv2.ini file to workaround this (see below).
To read the meta data with exiftool, open a cmd, try "pathToExiftool\exiftool.exe" "pathToCR3" | findstr /i "lens" and "pathToExiftool\exiftool.exe" "pathToCR3" | findstr /i "factor" which will reduce the output. The double quotes are only required, if there is white-space in a path.
Look for the reported combination of lens ID and crop factor and see, if it matches with the lensfun database. If not, you may edit the database and / or add a shortcut for darktable by providing an exiv2.ini file (see Exiv2 - Image metadata library and tools) and add an alias entry for the lens ID number as follows
[canon]
lensIdNumber=Tamron 10-24mm f/3.5-4.5 Di II VC HLD B023
As I’m using a Nikon, I can’t try this here on my side in full, but I hope that you got enough hints to solve the problem. Please also give the forum search a try, if you are still struggling… If the above mentioned method solves the problem, it may be an idea to track an issue for exiv2, lensfun or darktable on github depending on where the solution points to 
Good luck and have a wonderful x-mas time, Lars.
P.S.: assuming you are running latest version of dt, which is 5.0 as of yesterday.