I'm not used to that being missing from the web ui.
But these can also be set from the cli as well.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB5453
From the configuration > Config File menu
You can scan the command line options in place for this interface and see if it has been set explicity 100 / full. If it is not set that way in the commands then it is coming up this way during auto-neg.