Issues with enabling pg_stat_statements
In case of such issues it may be useful to check:

show shared_preload_libraries ;
select name,setting,source,sourcefile,sourceline,pending_restart from pg_settings where name = 'shared_preload_libraries';
Maybe something overrides your shared_preload_libraries somewhere or maybe server didn't actually restart, then 2. will show that as setting field will show something else than pg_stat_statements or pending_restart will show t if that change still waits for restart.

(i don't have comment rights here, so i post as answer hoping it will help)