Salesforce Packaging Bug with Lightning Components? (Warning: very strange!)

I was able to replicate this issue in two packages, so have raised it with support, and they are currently researching it (they were able to re-create it)

One thing I noticed as a workaround - I had many classes with mock callouts, both Web Service and HTTP. But only some failed - and they both did NOT make use of Start/Stop Test.

So even though they worked fine in regular test runs, once I updated my tests to invoke the code that invokes the callouts like below, I could then upload the package.

Test.StartTest() 
    //Invoke code that makes callout
Test.Stop Test()

For reference and tracking, here is the known issue:

https://success.salesforce.com/issues_view?id=a1p3A000000EO8L