This isn't a question but a solution. I have a couple of third party lens, Sigma and Tamron that Affinity Photo didn't automatically find in the lens database. The lenses were in the database and I found them quite easily and was able to select them but I don't search like a computer.
I found that the problem was that my Canon was recording the EXIF lens data a little different than the <model></model> element for the matching lens. As a human I knew that C and Contemporary are the same but a computer program can't make that same correlation.
If you are a little brave it is an easy fix.
Get the latest Lensfun file from Github.
The database folder "db" uses "mil-" for mirrorless cameras and "slr-" DSLR cameras. Make sure you pick the correct type and brand. For me I had to edit "slr-sigma" and "slr-tamron".
Edit the file(s) for each of your affected lenses using a plain text editor like TextEdit or Notepad. Find the correct lens. Pay attention to the crop factor. You will generally find 3 crop factors per lens. Make sure you edit the correct crop factor. Change the name found in the <model> element to match what Affinity Photo has in the top bar. It appears you don't have to worry about upper and lower case but if the model in the file is C but the EXIF data is Contemporary, change it to Contemporary. On my Tamron lens I had to add Macro to the end and get rid of the F infant of the 2.8.
Copy all files into the "Lens Profiles" folder. The easiest way to get there is to select "Open Lens Profile Folder ...". On a Mac it will say "In Finder", on Windows it will likely say "in Explorer". Copy or move the files you edited into this folder.
Relaunch Affinity Photo and open a raw file with one of your lenses that hasn't automatically selecting. If you did it right and matched the EXIF data, and selected the correct crop factor, Affinity Photo will now automatically find and apply the correct lens profile.
This has already helped me in my workflow since I no longer have to remember to apply the profile.
