RJChamp
We do support Instant Payments between bunq and Sparkasse in both directions. Unfortunately for certain Sparkasse IBANs we always receive a rejection back from their systems. To ensure the payment still arrives and isn't blocked completely, we then fall back on the old (slow) payment systems.
Why Sparkasse sends us a rejection I do not know. It seems to happen only on some IBANs, so it's possible they don't have incoming Instant Payments enabled for all their accounts. Maybe they can tell you more if you contact them.