I have a somewhat similar situation. I build and install both RT and ART from source code. I have lensfun 0.3.95-6 installed. I added manually entries for Nikon Z6 and Nikkor Z 24-70 f/4 S. ART can automatically recognize both body and lens, while RT shows only Z6, but not the lens. There must be some differences how RT and ART handle lensfun API.