Hi @mayorovp_3701,
This is a bug you should definitely report to GitLab -- it's clearly required by the API and all other registries support it. Without that API defined, we have to "make guesses" based on undocumented conventions (i.e. /v3-flatcontainer/{id}), which causes headaches down the line. That's why there's an API contract :)
I don't know how/why it works with dotnet sdk -- perhaps it's using a different endpoint, or maybe it's not actually working and just going to NuGet.org. Whatever the case, it doesn't make sense for us to build workarounds for easily-fixable bugs on GitLab when other vendors seem to follow the API contract.
-- Dean