TimeMachine Issues

Not going to lie, I have always found TimeMachine really hit and miss. It will work for months and then suddenly break with no changes to my system or config. One of these breakages has happened that I can't seem to figure out.

It happened once and I couldn't fix it so I started a new timemachine backup which worked for a week or so and then broke again in the same way.

I'm backing up to a synology NAS that has ~16tb total storage and 11tb free. Somehow its failing due to a lack of space. The TimeMachine volume is set up to allocate as much space as it needs as all the volumes on the NAS are virtualised and you can either set a limit or just allow the volume to scale as necessary.

The second thing that makes 0 sense is that I have a 512gb HD in the macbook pro backing up and it is doing an incremental backup yet wants 1.39tb of space.

So to summarise. Timemachine won't backup because it doesn't have enough space left even when there are 11tb of free space available and its also asking for 1.39tb of space to do an incremental backup on a 512gb HD.

Any ideas?

Oh and no drives attached and I am not backing up any virtual machines.