I have actually seen this issue now twice in a two week span at different clients with the same resolution so either it’s a common issue or it’s just a coincidence. In this scenario the database maintenance plan is set to delete old backup jobs, the job completes but the files are not deleted. In both cases the fix was simple, in the plan it asks you to specify the file extention it should delete which in most cases is bak. Make sure when you are entering this yyou enter JUST bak, not .bak or “bak” or anything else. If you have any charachters (including spaces) other than just the letters bak it will not work.
Hope this helps you