Cannot push gems: "Platform can't be blank"

[mattbrictson]

Steps to Reproduce

1. Attempt to publish a new gem version by running rake release (using the standard bundler/gem_tasks)
2. Enter OTP code
3. Push to rubygems fails: "There was a problem saving your gem"

Expected Behavior

I expect the "Pushing gem to https://rubygems.org" step of the release gem task to succeed.

Current Behavior

$ rake release
jess 1.4.2 built to pkg/jess-1.4.2.gem.
Tagged v1.4.2.
Pushed git commits and release tag.
Pushing gem to https://rubygems.org...
You have enabled multi-factor authentication. Please enter OTP code.
Code:   ******
There was a problem saving your gem: Platform is invalid, Platform can only include letters, numbers, dashes, and underscores, Full name can not end with a period, dash, or underscore, Platform can't be blank

Full list of errors:

• Platform is invalid
• Platform can only include letters, numbers, dashes, and underscores
• Full name can not end with a period, dash, or underscore
• Platform can't be blank

Environment

RubyGems version: 3.6.1
Bundler version: 2.6.1

Additional Context

It seems that something is causing the platform value to be interpreted as a blank value. My gemspecs do not explicitly set a platform, so I would expect the default platform to be used, as documented here: https://guides.rubygems.org/specification-reference/#platform=

Did something change recently that is causing the default platform value to no longer populate?