I am also seeing this behavior as well. These logs used to populate back in May which leads me to believe this was a bug introduced sometime in June. The last thread I have that included these output logs was on June 3rd.
Perhaps these log outputs are being hidden now for security reasons? Would love to get some input from OpenAI here 
Example of thread that had output logs: thread_ROsLxmQ4WWXkCx7crNoj0FJx
Another post that has the same issue: Why the outputs of code interpreter are EMPTY? - #4 by 23randomforest