TSQL window function to calculate interest

I am trying to calculate interest using window functions on SQL Server 2022

with t as 
(
	select * 
    from 
        (values
            ('2000-1-1',100,0.1),
            ('2000-1-2',100,0.1),
	        ('2000-1-3',100,0.1),
            ('2000-1-4',100,0.1),
	        ('2000-1-5',100,0.1),
	        ('2000-1-6',100,0.1)
        ) as t(date, amount, perc)
)
select 
    *, 
    perc * sum(amount) over (order by date rows between unbounded preceding and 1 preceding) / 365 interest,
	sum(amount) over (order by date rows between unbounded preceding and current row) +
		perc * sum(amount) over (order by date rows between unbounded preceding and 1 preceding) / 365
from 
    t

Can someone point me to a problem why interest calculated isn't being cummulated f.e. I expect balance to be 300.08 on 3rd while it is 300.05 (interest from 2nd isn't taken into account)

As mentioned in the comments, this isn't cumulative aggregation it's recursive aggregation. Effectively your expression wants to read something like:
Amount + PriorBalance + (Interest + (Percentage * PriorBalance) / 365)

To achieve this, you're doing to need to use a recursive CTE, as a cumulative SUM (or any of the windowed aggregated) cannot access the prior rows calculated value.

Therefore this appears to be the solution you need:

WITH t AS (
    SELECT date,
           amount,
           perc,
           ROW_NUMBER() OVER (ORDER BY date ASC) AS I
    FROM (VALUES(CONVERT(date,'20000101'),100,0.1),
                (CONVERT(date,'20000102'),100,0.1),
                (CONVERT(date,'20000103'),100,0.1),
                (CONVERT(date,'20000104'),100,0.1),
                (CONVERT(date,'20000105'),100,0.1),
                (CONVERT(date,'20000106'),100,0.1)) as t(date,amount,perc)),
rCTE AS (
    SELECT date,
           amount,
           perc,
           I,
           CONVERT(numeric(7,6),0.000000) AS interest,
           CONVERT(numeric(12,6),0.000000) AS balance
    FROM t
    WHERE I = 1
    UNION ALL
    SELECT t.date,
           t.amount,
           t.perc,
           t.I,
           CONVERT(numeric(7,6),r.interest + ((t.perc * r.amount)/365)) AS interest,
           CONVERT(numeric(12,6),t.amount + r.balance + (r.interest + ((t.perc * r.amount)/365))) AS balance
    FROM t t
         JOIN rCTE r ON r.I = t.I -1
)
SELECT date,
       amount,
       perc,
       interest,
       balance
FROM rCTE;
GO