Apparently your problem starts here:
> -> Function Scan on filter_b_preke_matoma (cost=0.00..267.50 rows=5 width=126) (actual time=6.580..11.766 rows=2820 loops=1)
> Filter: (((prek_pavadinimas)::text ~~* (('%'::text || ($3)::text) || '%'::text)) OR ($3 IS NULL))
Planner expects to see only somewhat 5 rows after function scan with the filter but get ~3000, which is not a surprise if one looks at your plain SQL query, corresponding WHERE part:
AND ((prek_pavadinimas ILIKE ('%'||null||'%')) OR null is NULL)
As I mentioned conditions like this get wrapped (to TRUE in your case), so with plain SQL planner does not even try to estimate ILIKE filter effect.
