I have the same issue when it comes to PTO requests. I have opened 2 tickets in the past regarding this issue; case# 00308908 and case# 00243922 if you would like to reference them. I was told there was nothing I could do about it as allowing everyone to view this potentially sensitive information is built-in to Workfront.
I had to send an announcement stating that Workfront had insufficient confidentiality controls and to be very careful what you put in a request, especially the reason for a time-off request.