Thanks gents!
I thought I’d caught them all, but I still had an UmbracoApiController hidden in the project which was causing the memory spiral. Change that out to a Controller and we’re laughing (reference doc here if anyone needs it).
All signs point to Swagger auto-gen, which would definitely back up the case of not running when in dotnet run. Still wouldn’t have thought it should cause a memory leak, I’ll see if I can screw it up in a vanilla and log the bug if so.