It seems that Octopus Deploy 3.12.0 cannot read a nuget feed created with nuget.exe init or add which creates a specific folder structure (see https://docs.microsoft.com/en-us/nuget/tools/nuget-exe-cli-reference#init).
Could you confim it is actually the case and is there any way to fix it ?
Sorry about that, it is a bug. I have raised an issue (https://github.com/OctopusDeploy/Issues/issues/3625) for it, for now given that is just a local folder, so instead of using nuget.exe init + add, just copy the packages to a folder and register that folder as a nuget feed.
Hope this makes sense.
I will change our repository if this is the only solution and I will keep an eye on the bug to know when it will be fixed.