CREATE TABLE ORG_ACCOUNTS
(
    ORGACCOUNTID BIGINT NOT NULL PRIMARY KEY
);

CREATE TABLE BALANCES
(
    BALANCEID BIGINT NOT NULL,
    ORGACCOUNTID BIGINT NOT NULL,
    BALANCEDATE DATE NOT NULL
);

ALTER TABLE BALANCES ADD CONSTRAINT PK_BALANCES PRIMARY KEY (BALANCEID);
ALTER TABLE BALANCES ADD CONSTRAINT FK_BALANCES_ORGACCOUNTS FOREIGN KEY (ORGACCOUNTID) REFERENCES ORG_ACCOUNTS (ORGACCOUNTID);
ALTER TABLE BALANCES ADD CONSTRAINT BALANCES_BALANCEDATE_ORGACCOUNT UNIQUE (ORGACCOUNTID, BALANCEDATE);

CREATE DESCENDING INDEX BALANCES_BALANCEDATE_DESC ON BALANCES (BALANCEDATE);

select first 1 *
from Balances B
where OrgAccountID=18 and
   BalanceDate<='01.01.2017'
order by BalanceDate desc;
 
v2.5:
PLAN (B ORDER BALANCES_BALANCEDATE_DESC INDEX (BALANCES_BALANCEDATE_ORGACCOUNT))

v3.0:
PLAN (B ORDER BALANCES_BALANCEDATE_DESC)

Correct (best) plan should be:
PLAN (B ORDER BALANCES_BALANCEDATE_DESC INDEX (FK_BALANCES_ORGACCOUNTS))

Now the funny thing:
ALTER TABLE BALANCES DROP CONSTRAINT BALANCES_BALANCEDATE_ORGACCOUNT;

v3.0:
PLAN (B ORDER BALANCES_BALANCEDATE_DESC INDEX (FK_BALANCES_ORGACCOUNTS))

I.e. existing compound index BALANCES_BALANCEDATE_ORGACCOUNT is not only ignored itself (see also CORE-5070), but it also hides possibilities to use another index FK_BALANCES_ORGACCOUNTS.