If it was working correctly and you have changed nothing regards settings etc then it can only really be a mechanical issue. It will either be a slipping motor coupling or the Axis is binding up causing the lost steps.
If it's a different file with different settings then you could just be pushing the machine too hard. To test just lower the feed rate and see if it happens.

It could possibly be electrical noise but It would be random and would have happened all the time.

If not settings etc then I would start by marking the motor coupling and motor shaft to see if you got any slippage.