I've had the same issue. But with mine sometimes it works and sometimes it doesn't. I did notice that if there isn't enough room on the drive it will not delete the previous files.
What I did is create a job that deletes the backups everynight at 6pm. By 6pm the Enterprise backup has already taken the backups off of the server.
It's very simple. Create a batch file that does a del *.bak and call the batch file from the job step.
We found that if you have a data base using the SIMPLE model but still do a transaction log backup then this feature may not work for ANY of the databases in the same maintenance plan. The number of backup files just grew and grew.
I found that same issue with the transaction log bkup. Microsoft ...
If you have the time everyday and remember to do it, I guess a manual bkup is better.
On my end, we update our databases from our old cobol dos system with txt files so if we loose a day it's not a big deal. We can just reload the data.
The other option would be to talk to who ever is doing the enterprise bkups off of the server. Maybe they can move instead of copy the files.