Ember build error: Assertion Failed: [BUG] seen set should be available

Trying to update a Rails/Ember app and running into this error:

Template Compiler Error (broccoli-persistent-filter:TemplateCompiler) in ember-basic-dropdown/templates/components/basic-dropdown/content.hbs

Assertion Failed: [BUG] seen set should be available

Googled far and wide, but have not been able to find a fix. The only thing I found was this GitHub issue. Downgrading to 3.20.0 did not solve the issue.

Relevant packages:

"ember-cli": "^3.27.0",
"ember-data": "~3.26.0",