Hi Krystian, I did contacted PayPal Merchant Support.
But they couldn’t really solve the problem, they said the “INSTRUMENT_DECLINED” issue was the problem of buyer’s side (or bank/card issuer).
However, after I did a deep clear cache on both SiteGround Admin and WordPress Admin (FlyingPress plugin & SiteGround Optimizer), I found that the issue “INSTRUMENT_DECLINED” has decreased significantly!