1) I checked: snapshots on APFS DO TAKE EXTRA SPACE!
2) Ordinary users like me are not allowed to report on 'bugreport.apple.com';
if you can, you're welcome.
;JOOP!
Of course you can report at bugreport.apple.com, just a normal Apple ID will let you login and report, I did that many times already.
Time Machine snapshots on APFS volumes don't take any extra space. If you really want to talk to Apple, go to bugreport.apple.com
The Local backup itself should not occupy too much extra storage IF the original is still there.
AFAIK, the logic is a bit complicated.
e.g. If you have a 1GB file on your SSD. It will of course occupy 1GB on your SSD. However, local TM should not take any extra space other than making a pointer let the TM local backup "share use" original file's 1GB data. That means, effectively still only take up 1GB space with your original file + TM local backup.
However, if you delete that 1GB file. Now problem comes. Because the original file is no longer there. But the TM local backup need it (the pointer still there). APFS will not free up the space even though you empty trash, but let that 1GB data now belongs to the local TM backup. At this point, I personally will consider TM local backup take up 1GB disk space.
So, for a user that has lots of files movements. Their local TM backup can getting large quickly and occupy lots of disk space even with APFS.