From State/To State empty in v1.9 development version

Did you run update.php.
Version 1.8 and 1.9 are about better usage of baseFieldDefinitions.
This also includes from state and to state.
If update.php does not work, you should remove and add the fields to the view.

drush updb did the trick. My bad, I'm reloading the database too much in ddev.

This issue may be closed. What I did find while checking the base field definitions is that v1.8 has ->setSetting('target_type', 'workflow_state') on the from_sid/to_sid fields in the BaseFieldDefinitions. Version 1.x-dev doesn't have that, oversight or intentional?

That seems like an oversight. Let me check.

The link is a 'list_string', in order to get proper widgets for this.

I might revert to entity_reference, but that will get 'autocomplete' widgets, too, in the workflow settings.

If you use a Views display, the from_state and to_state fields must be deleted and added again, since the key is changed from 'target_id' to 'value'.
(but let me test)

Regarding your remark #3.
I did change the $transition->to_sid and $transition->from_sid baseFields from 'entity_reference' to 'list_string'.
'list_string' does not need ->setSetting('target_type', 'workflow_state') .
ITMT, also because of your remark, I returned to the original, better 'entity_reference'

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