Cannot add class to Text Long format fields

I've tried (and failed) to replicate this with the following field types. The module worked for all of them, using all available formatters from Drupal core.

• Text (formatted long)
• Text (plain long)
• Text (formatted, long, with summary)

Can you provide more details of how your Bio field is configured please? It would be helpful if you could export the following via the config UI at admin/config/development/configuration/single/export:

• "Field Storage" config for Bio field
• "Field" config for Bio field on the node( or user, or other entity) bundle
• "Entity view display" config for the relevant display where you fail to set the field formatter class.

Also, in your 6.12.07 screenshot, I don't see a settings gear-wheel button. Can you confirm whether you see that? I'm not sure if your screenshots are cropped.

Thanks @andreas.

I tested all three long text field types on User profiles, and still can't reproduce the problem. (It was worth mentioning users rather than nodes though. For a brief moment I wondered if it was some problem with bundle-less entity types, but I think that can be rules out.)

The screenshot in #3 is interesting. It differs from the Manage Display UI in Drupal 8 core in a number of ways, with a mix of formatter and widget terminology:

• The table headers include a separate column headers for Field, Widget, and Operations. In D8 core there is a just a "Format" header with a colspan of 3.
• The table has a "widget" column header, but D8 core calls it "format". Nonetheless, the open settings say "Format settings" as expected.
• The UI shows regions (top, content) so some kind of field layout module is clearly in use. However it also shows a "region" column with select elements for choosing a region. The core field layout experimental module does not have the region column (at least in drupal-8.3.1), so that suggests that some other module is in use.
• The button from Field Group module is shown.

What other modules are enabled here? (I'm guessing Display Suite and Layout Plugin.) Can you provide a list of modules enabled, together with version number (and D8 core version). Note that Display Suite needs to be the correct version to match whichever D8 core version you use.

At this stage I suspect there might be some incompatibility between third party settings API and Display Suite (or Layout Plugin), depending on how they alter core Field UI. Field Formatter Class is the most popular of the modules using third party settings API, so it makes sense the issue would surface here.

Since this issue was 5 years ago and could never be reproduced, closing as such.