Suddenly can't compile for v6.0, even thought I've just tested it on a 6.0 emulated device?

Good morning,

I've just finished writing my project and testing it on an emulated device with v6.0 (which worked nicely),
but apparently magic happened and after clicking 'Archive', I can no longer compile or deploy my app to anything lower than 8.1?

I get these errors:

The $(TargetFrameworkVersion) for RSH.Android (v6.0) is less than the minimum required $(TargetFrameworkVersion) for Xamarin.Forms (8.1). You need to increase the $(TargetFrameworkVersion) for RSH.Android.

The "ResolveAssemblies" task was not given a value for the required parameter "ReferenceAssembliesDirectory".

Despite the fact, I've been running it on 6.0 previously without a single issue.

Is this a bug or have I by accident referenced a more up-to-date library before I've hit 'Archive'?