Thank you for your contacting us with this issue.
The reason for this issue is that the parsing process failed at one of the files in your project during its opening.
This is some part of your code (some method, condition or even variable declaration) that cannot be handled by the parser.
It fails and doesn't go further through other Apex Class files. This is why the Code Assistance option and the Code Outlining are absent for these files.
The same reason is actual for the absent tests. They are not recognized as tests in your code.
Currently, our developers are working on this issue.
All the cases of the failed work of the parser that are known for us are already covered and we are looking for other ones.
This can take some time.
You can use a workaround for your work with tests: please open the 'Test Results' panel, click 'Run Tests' and then click 'Refresh List'. This is the query to Salesforce to get all the test from you Organization.
Please let us know if this workaround is acceptable for you.
I'll keep you updated when the issue related to the absent Code Assistance will be solved.