Merged 10768aa2 and seeing the error again under app/controllers/:
method=GET path=/api/v1/timelines/public format=*/* controller=Api::V1::Timelines::PublicController action=show status=500 error='NameError: uninitialized constant ActiveModel::Type::Text'
application_controller.rb:76:in `cache_collection'
api/v1/timelines/public_controller.rb:22:in `cached_public_statuses'
api/v1/timelines/public_controller.rb:16:in `load_statuses'
api/v1/timelines/public_controller.rb:9:in `show'
Clearing the cache seems to have fixed the problem.
$ heroku run rails c -a zundan-mastodon
irb(main):001:0> Rails.cache.clear
=> nil