SELECT inside trigger gives wrong result (it ignores the WHERE clause)
I've just tested this with MariaDB 10.4.12 with innodb_version 10.4.12 on Fedora Linux, but I don't see the problem you have - I get an empty query3.csv file. The only difference I have (besides the Linux distro) is that I don't have your foreign key constraints since I don't have those tables.

Are you sure you're looking at the correct output file? 

Note also that you will get a warning when you run the query or create a trigger with that query:

SHOW WARNINGS;
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                               |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1287 | '&lt;select expression&gt; INTO &lt;destination&gt;;' is deprecated and will be removed in a future release. Please use 'SELECT &lt;select list&gt; INTO &lt;destination&gt; FROM...' instead |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.000 sec)


So you could try moving the FROM and WHERE clauses to the end of the statement. I get the same result with this, just without the warning. 

You can also try enumerating the columns in the SELECT instead of SELECT *, just to see if it changes anything.

This is my trigger:

CREATE TRIGGER trigger1 AFTER UPDATE ON stock_totals 
  FOR EACH ROW 
    SELECT * FROM `stock_totals` 
    WHERE `product_id` = 2 AND `stock_place_id` = 3 
    INTO OUTFILE '/tmp/query10.csv'  
    FIELDS TERMINATED BY ','
    ENCLOSED BY '"'         
    LINES TERMINATED BY '\n';