Redmine

As a workaround, we also tried to make the "Assignee" field mandatory/required. But that didn't seem to work either: there is no red star appearing, most likely indicating the field is not actually recognized as mandatory.

the assignee is cleared when the validation fails

I cannot reproduce. Could you describe the exact conditions/steps in order to reproduce?

We use custom fields which are defined to be mandatory. The following steps reproduce the problem:

• Create a new issue and fill in all details except for a mandatory custom field (in our case a dropdown which still says "--- Please select ---")
• Choose create

As a result, you will get a validation error saying "<Field> can't be blank". In addition, the "Assignee" field will be cleared (set to blank) or to the default assignee if one is set.

I can't reproduce it either on 2.3.1.stable.11915 (m.redmine.org). The behavior is the same as described by Jean-Philippe; the assignee is preserved after the validation error.
Do you have any plugins installed?

Martin Studer wrote:

[...] or to the default assignee if one is set.

With that you mean default assignees for issue categories? Otherwise you are using a plugin which probably also causes this issue.

Right, we make use of the redmine_default_assign plugin. I guess that is causing the issue then.