This conversation has been merged. Please reference the main conversation: Lightroom: Set Maximum # Of Catalog Backups, and Auto-Replace Oldest
That actually feels like a huge bug, but seems to be a "feature".
So here to request would be what tons of other pieces of software already do:
Make it an option to discard old backups automatically via one of two criteria:
a) discard oldest backups if total sum of backap data exceeds x megabyte
b) discard oldest backups if the get older than x days/weeks/months