I’m afraid I haven’t been able to reproduce this issue. When I test here, pressing the “Delete” key reliably deletes all selected playlist entries, regardless of the method used to select them.
I may be missing some important detail or step of the process. What operating system do you use?
Can you confirm that all the audio table entries are highlighted in gold after you press Ctrl+A?
Are you able to record a video of this issue and upload it to your profile page?
After some more investigation, I discovered that the bug occurs in FMOD Studio version 2.03.12, but not in the unreleased version 2.03.13 that I was testing with when you reported the issue, hence my inability to reproduce it.
Which is to say, the issue has been fixed. I recommend downloading FMOD Studio version 2.03.13 when it is released.