read_only fields longer the 255 characters accepted and render as null

Issue Summary

As reported in https://community.developer.atlassian.com/t/read-only-issue-field-returns-null-instead-of-contents-of-linked-json-property/45247, it appears that read only fields are limited to 256 characters in length. It should not be possible for Jira to accept longer field values such that it incorrectly rendered the field as null.

Steps to Reproduce

1. Create a read_only issue field.
2. Set a value that is 256 characters or longer in length.
3. Define a filter that includes the isue.
4. View the filter in list mode with the field visible.

Expected Results

At step 4, the field renders as expected. Alternately, at step 2 the request to set the field is rejected.

Actual Results

At step 4, the field renders as "null". 

Workaround

None