This is reproducible on Data Center: yes
This behaviour is reproducible since from Jira version 9.x and not reproducible until version 8.22.x.

Issue Summary

Error 405 was received after pressing ENTER key in the empty Watchers field.

Steps to Reproduce

1. Navigate to Issue view screen
2. Click on "Watchers" > "Add Watchers" Field
3. Pressing ENTER in this empty field will throw the error: "HTTP Status 405 – Method Not Allowed"

Expected Results

When the field "add watchers" does not have a value assigned and while pressing enter, the issue view page screen to be reloaded.

Actual Results

The following error is thrown:

In the application-jira.log, we can see the following warning message:

2023-03-21 06:46:38,024+0100 https-jsse-nio-8443-exec-1665 WARN x /browse/ABC-123 [c.a.j.web.dispatcher.JiraWebworkActionDispatcher] Action 'com.atlassian.jira.projects.issuenavigator.webaction.ProjectIssueNavigatorAction!issueViewWithSidebar' does not support 'POST' request method

Workaround

Currently there is no known workaround for this behaviour. A workaround will be added here when available