When using the duotone feature, picking a custom color shows up properly in the editor view, but when you publish, the filter does not appear. However, the presets work just fine. You have to change one of the colors to replicate this bug.

Steps to replicate:

1. Select image block with photo
2. Apply duotone filter
3. Change one or both of the colors
4. See colors change in editor
5. Press publish
6. Navigate to front end of site
7. Notice photo now has no filter applied

Screenshots

Editor view:

Published view:

Have tested with a variety themes and versions of Gutenberg. Consistent results. @annezazu can also replicate.

cc @ajlende