Commented by: @pavel-zotov

Consider two scripts from attached file ("c1364-wrong-result-when-connect-using-utf8-charset.zip").

1) When I run in command prompt test that uses ISO8859_1 as connection charset then all works fine:
C:\MIX\firebird\QA\fbt-repo\tmp>chcp 1252
<current code page is>: 1252

C:\MIX\firebird\QA\fbt-repo\tmp>C:\MIX\firebird\fb40\isql -q -i c1364-iso8859.sql 1>c1364-iso8859.log
(log see in attach)

2) But when i change code page to 65001 (somewhat like utf8 for Windows) and use UTF8 as connection charset than almost half of required output is missed.
This is how iit can be done:

C:\MIX\firebird\QA\fbt-repo\tmp>chcp 65001
<current code page is>: 65001

C:\MIX\firebird\QA\fbt-repo\tmp>C:\MIX\firebird\fb40\isql -q -i c1364-utf8.sql 1>c1364-utf8.log
File c1364-utf8.log also see in attach.

Is it expected ?

PS.

One may to check result of following statements rather than downloading zip and run test scripts:

select 1 from rdb$database where _iso8859_1 'àâçéèêëîïôùûü' collate fr_fr_ci_ai like '%î%';
select 2 from rdb$database where _iso8859_1 'ÀÂÇÉÈÊËÎÏÔÙÛÜ' collate fr_fr_ci_ai like '%Ô%';

If you will run them using UTF8 charset then result will be empty (no rows).