In Drupal 8, the teaser text for a long form text field (specifically, text of type "long, formatted, with summary") does not have a CKEditor wrapper around it:
How can CKEditor be added to this textarea?
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This is core limitation right now.
I would recommend that if you want an editor on a teaser/summary field, you simply create a separate field, that also gives you options like having a separate text format for it.
The text long with summary field type might eventually get removed in Drupal 9 or so.