This ended up being due to a global invocable action being included in a managed package that we no longer subscribed to (for us, it was TaskRay). This will happen with any license restricted managed package with invocable actions. Additionally, the issue will only show itself in a production instance since licenses aren't managed in a sandbox.

Salesforce published a knowledge article to explain but has not, as far as I know, published a known issue.

https://help.salesforce.com/articleView?id=Error-Can-t-load-invocable-actions-in-visual-flow&language=en_US&type=1