You will not be able to add umbrella or collection projects, they pick up observations automatically. You won’t see them listed under the projects heading for the observation view unless you “join” those projects.
This next bit is my understanding, the actual implementation may vary slightly: When you go to select a project, you will be offered a list based on projects you have joined (and never umbrella or collection projects, see above). If you start to type a project name, then the list gets rebuilt, and as you get further into the name it starts to drag in project names that you haven’t signed up for. Sometimes the trigger for this rebuilding of the list gets a little out of sync, in which case just pause briefly, then type one more letter of the name, and the list should build correctly.
If it still doesn’t behave as you expect, copy some urls of project and/or observations and paste them into your reply here in the forum and state what it is that you expect to happen, there are many of us that are happy to take a look and see what the problem might be.