I'm not an expert, but speaking from my personal experience here. I was in a similar situation a while ago with my Mac Pro 5,1 running Mojave. Normally when you attempt to download iMovie on Mojave, you are prompted with something that says "This version of iMovie is unsupported on your Mac, would you like to download an older version?". Though when I attempted this with my Mac, I got another message that said I did not "Own" iMovie, and could not download it.
Fast forward to January, I bought a new 2020 M1 MacBook Air, and that came with iMovie preinstalled as part of the included software suit. Cool, but I still want it on my Mac Pro if I could. So now I was able to see iMovie as an owned application. So I tried to download it again on my Mac Pro. However at that point in time, there was an issue with the App Store in Mojave not working properly. I was finally able to download it today.