Using Moodle 3.8.2+ is not possible to update a existent forum idnumber by using the form at the URL https://mymoodledomain.com/course/modedit.php?update=<forum-id>&return=1

No errors are raised, so I think the update code is not considering this field as other fields are being saved correctly.

Steps to reproduce:

1. Create a new forum
2. Let the idnumber empty
3. Update the forum and set a idnumber
4. Check to see if it was saved

I did a clean install, my environment is (everything updated):

• Ubuntu 18.04
• Nginx
• PHP-FPM 7.2
• Mysql 5.7
• Moodle 3.8.2+ (latest)