Prices can only ever be copied between the 01.01. and the 31.12.
It is not possible to copy price bands that overlap a year.
This means, if your price band starts in one year and ends in the next, no prices past the 31.12. will be copied.

The graphic above illustrates what is happening.
When copying past the 31.12. the system would find two price bands to be implemented in January.
The one from January of the year to be copied (2020) and also the January of the price band which is reaching past the end of year (2021).