MariaDB 10.3

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select version();

version()

10.3.36-MariaDB-1:10.3.36+maria~deb10-log

create table mytable (id int, details json);
insert into mytable (id, details) values 
  (9, '[{"amount"  : 100,  "flag"  :  true, "fieldA"  : "abcd"},
 {"amount"  : 101,  "flag"  :  false, "fieldB"  : "bcde"}, 
 {"amount"  : 103,  "flag"  :  true, "fieldA"  : "abcd"}]'),
(10, '[{"amount"  : 110,  "flag"  : false, "fieldA"  : "abcd"}, 
{"amount"  : 102,  "flag"  : true, "fieldB"  : "bcde"}]');

Records: 2  Duplicates: 0  Warnings: 0

select sum( json_value( t.details, concat('$[', n.i, '].amount') ) ) total_amount
from mytable t
inner join (select 0 as i union all select 1 union all select 2) n 
    on n.i < json_length(t.details)
where json_value( t.details, concat('$[', n.i, '].flag') ) 

total_amount

305