Steps to reproduce:
Have item which measures storage in bytes and has value over 1TB
(1TB = 1 * 1024 * 1024 * 1024 * 1024 = 1,099,511,627,776)
Create calculated item (data type: number(unsigned)) using forecast function.
You will get value: 999999999999
Yes! it's documented behavior
and
Yes of course you can use a workaround like introducing preprocessing step which divides values by 1024 and you operate with KB or even do that again to get values in MB resulting in:

1. previous history and trends becoming useless;
2. Automatic units calculation = useless.

And our days storage reaching TB, PB of data or even more is normal thing.