I think this is strange , so far I know developer should be able to publish and this means also to create a new widget (Vuforia Studio) Because Studio creates mostly the widgets offline (only if there are any twx data will try to update but it will still continue also when the connection fails) therefor I think the described situation is a little strange.
But when first time I tested it , I was able to reproduce the problem.... but actually it was caused because I tried the publish an experience which are create as admin user . This means if the admin user created some TWX entities or studio widget then seem to have a problem
When I created a new experience with a user which is member of the developer Thingworx group then it was working. I was able to publish first time. Then add a new widget and republish it again - this worked
I tested with Vuforia Studio 8.5.5, Experience Service ("version":"8.4.0-b792") and ThingWorx 8.4.1-b2126
I tested with the following steps:
-added an user e.g. developer to the members of developer user group
Then I created a new experience as developer user (member of TWX developer group) .
Published it and then created a new widget,
saved it and republished it again -this was working:
So when I check the project on the server - I could see the new widgets are updated:
Because I was connected to the Experience Service as admin to open some files for check , then one time I received an error and publish did fail.
... But when I unpublish the experience from servier (as developer)
Afterword’s it was working again - I was able to publish the modified experience / Studio project.
So I believe your issue is caused mainly when the experience project is edited also form users which are members of the admin group and published. Or in some case it access Thingworx entities in the External data section where the developer group user has no write permission for the design time