MariaDB DATETIME Index not working with Between FROM_UNIXTIME() - datetime

Question

I have a table with DATETIME field, which is indexed by a BTree. Now i want to query it with following statement:
SELECT
count(us.CITY) as metric,
us.CITY as Name,
us.LATITUDE as latitude,
us.LONGITUDE as longitude
FROM
FACT
LEFT JOIN
USER us
ON
us.ID_USER = FACT.USER
WHERE
ASSESSMENT_DATE BETWEEN FROM_UNIXTIME(1601568552) AND FROM_UNIXTIME(1604028277)
GROUP BY us.CITY, us.LATITUDE, us.LONGITUDE;
EXPLAIN:
+------+-------------+-------+--------+----------------------------+---------+---------+------------------------------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+--------+----------------------------+---------+---------+------------------------------+--------+----------------------------------------------+
| 1 | SIMPLE | FACT | ALL | INDEX_FACT_ASSESSMENT_DATE | NULL | NULL | NULL | 762621 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | us | eq_ref | PRIMARY | PRIMARY | 46 | dwh0.FACT.USER,dwh0.FACT.ENV | 1 | |
+------+-------------+-------+--------+----------------------------+---------+---------+------------------------------+--------+----------------------------------------------+
2 rows in set (0.001 sec)
Interestingly, by only changing the dates manually into the DATETIME Format string it uses the index. But the FROM_UNIXTIME() function should in my opinion return the exactly same thing...
SELECT
count(us.CITY) as metric,
us.CITY as Name,
us.LATITUDE as latitude,
us.LONGITUDE as longitude
FROM
FACT
LEFT JOIN
USER us
ON
us.ENV = FACT.ENV AND us.ID_USER = FACT.USER
WHERE
-- ASSESSMENT_DATE BETWEEN FROM_UNIXTIME(1596649101) AND FROM_UNIXTIME(1599108827)
ASSESSMENT_DATE BETWEEN '2020-08-05 11:30:11.987' AND '2020-09-03 11:30:11.987'
GROUP BY us.CITY, us.LATITUDE, us.LONGITUDE;
EXPLAIN:
+------+-------------+-------+--------+----------------------------+----------------------------+---------+------------------------------+--------+--------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra
|
+------+-------------+-------+--------+----------------------------+----------------------------+---------+------------------------------+--------+--------------------------------------------------------+
| 1 | SIMPLE | FACT | range | INDEX_FACT_ASSESSMENT_DATE | INDEX_FACT_ASSESSMENT_DATE | 5 | NULL | 132008 | Using index condition; Using temporary; Using filesort |
| 1 | SIMPLE | us | eq_ref | PRIMARY | PRIMARY | 46 | dwh0.FACT.USER,dwh0.FACT.ENV | 1 |
|
+------+-------------+-------+--------+----------------------------+----------------------------+---------+------------------------------+--------+--------------------------------------------------------+
2 rows in set (0.001 sec)
Can anyone refer to such a problem? the where clause is generated by grafana, so i can not change that, but the rest i can change if it changes something.
Thanks for suggestions!

Answer 1

Sorry for bothering.. after around 10^5 more inserts, it works for both cases... Maybe it was just bad luck