Alias "U" is not available in the CTE "ZAWIESZENIA".
If you remove whole WHERE from the main query then an error is raised.
--------------------------
SQL error code = -206
Column unknown
U.UMOWA_DATA_DO
At line 8, column 190.
---------------------------

but if you add whole where no error present.
It can be releated somehow with window functions.

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WITH DATY_ZAWIESZEN AS
    (
        SELECT date '2020-01-01' AS DZ_OD, date '2020-01-31' AS DZ_DO FROM RDB$DATABASE
    )
    , ZAWIESZENIA AS
    (
        SELECT W_UZ.UMOWA_ID,
               U.UMOWA_DATA_DO /* ------------ ALIAS "U" IS NOT AVAILABLE HERE!!!! ------------ */ AS ZAWIESZ_DATA_DO
          FROM (SELECT 1 AS UMOWA_ID FROM RDB$DATABASE ) W_UZ
               INNER JOIN DATY_ZAWIESZEN W_DZ ON 1=1
    )
SELECT U.UMOWA_ID
  FROM (SELECT CURRENT_DATE AS UMOWA_DATA_DO, 2 AS UMOWA_ID FROM RDB$DATABASE) U
        INNER JOIN DATY_ZAWIESZEN DZ ON 1=1

/* ------------ without this WHERE an error is raised about unknown column UMOWA_DATA_DO ------------ */
    WHERE
        EXISTS
        (
           SELECT *
             FROM ZAWIESZENIA UZ
            WHERE UZ.UMOWA_ID = U.UMOWA_ID
        )