Tapioca not generating correct RBI file for Rails model

I'm migrating the way I generate RBI files from Sorbet to Tapioca. However, I am getting a recurring problem across a number of the RBI files for my Rails models. The RBI files seem to be tripping up when trying to produce a signature for a has_many relationship.

(Note - it's the RBI file itself rather than the model that is generating the warning)

In the user.rbi file, the signature for the user.plans has_many association is:

    sig { returns(::ActiveRecord::Associations::CollectionProxy[Plan]) }
    def plans; end

When I run srb tc I see the following error:

sorbet/rbi/dsl/user.rbi:110: Method [] does not exist on T.class_of(ActiveRecord::Associations::CollectionProxy) https://srb.help/7003
     110 |    sig { returns(::ActiveRecord::Associations::CollectionProxy[Plan]) }
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  Got T.class_of(ActiveRecord::Associations::CollectionProxy) originating from:
    sorbet/rbi/dsl/user.rbi:110:
     110 |    sig { returns(::ActiveRecord::Associations::CollectionProxy[Plan]) }
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  Did you mean:
    sorbet/rbi/gems/activerecord@6.1.4.1.rbi:3238: ActiveRecord::Delegation#[]
    3238 |  def [](*args, &block); end