TimeMachine says 'Back up xxGB' not 'Backing up xx of yy'

After 2 days this backup finally completed, going over 17 GB. Along the way it stopped a few times and warned of an error. I spoke to Apple Support. They screen shared to actually witness the problem, then after a long wait whilst it was discussed internally, I was told to let it complete.

The saga continues. After a few successful backups, Time Machine has now said it is going to re-build the backup (can't quote the exact words) but nothing will be lost. That's OK, but it involves backing up 338 GB onto a disk with only 396 GB available (out of 1TB).

It has spent half a day connected so far and managed 16 GB, so it might take 10 days, and in that time it says no new backups will be performed.

This is my laptop which is not on all the time, gets moved away from its link to the Time Machine disk, and often runs low on power.

What a saga. It has now done nearly half of the Backup, because I have switched to wired ethernet for faster transfer whilst connected.

BUT, I have lost all my previous backups and can't now go back in Time Machine.

So it seems that phrasing 'Back up xx GB' instead of 'Backing up xx GB of yy GB' was a precursor to losing all my history.

If I had known that I could perhaps have taken different action to preserve the history.

For clarification, there is still over 300GB of free space on the TimeMachine disk.