.NET Web API 3.1 fails on deploy for Linux

[fiveisprime]

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(From internal customer):

dotnet new webapi // 3.1 dotnet publish -c Release -o ./publish

right-click deploy, choose runtime latest

Gives error in logs like so:

2020-02-26T13:17:07.246774534Z Trying to find the startup DLL name...
2020-02-26T13:17:07.247350249Z Found the startup D name: 02demo-azure-app-service.dll
2020-02-26T13:17:07.247844261Z Running the command: dotnet "02demo-azure-app-service.dll"
2020-02-26T13:17:07.264710698Z It was not possible to find any compatible framework version
2020-02-26T13:17:07.265168010Z The specified framework 'Microsoft.AspNetCore.App', version '3.1.0' was not found.
2020-02-26T13:17:07.265524219Z - The following frameworks were found:
2020-02-26T13:17:07.265827727Z 3.0.0 at [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
2020-02-26T13:17:07.266045632Z 
2020-02-26T13:17:07.266412842Z You can resolve the problem by installing the specified framework and/or SDK.
2020-02-26T13:17:07.266634048Z 
2020-02-26T13:17:07.266644648Z The .NET Core frameworks can be found at:
2020-02-26T13:17:07.292463216Z - https://aka.ms/dotnet-download

[ejizba]

I'm not convinced "Latest" and "LTS" are running 3.1. The portal doesn't even give those as options:

There's clearly something up because here is our list of runtimes (which we do not hard-code):

[ejizba]

If I create a .NET Core 3.1 app from the portal and deploy from our extension it works.

[ejizba]

Per offline discussion, the platform team is working on updating the availableStacks api