Does not index new boolean fields on existing content

When indexing content, if a boolean field doesn't have a value on the node (i.e. it's null) then this should be indexed as false in Solr.

This expectation is wrong. Null means that the field is not stored in the index!
You need to write an update hook that updates your existing code to set the field value to false.
But that's standard Drupal.

Automatically closed - issue fixed for 2 weeks with no activity.

That doesn't make any sense.

Why wouldn't you just be able to filter on: Not equal to - True?