This is a very common experience for adult learners who reach high fluency: Dutch friends and acquaintances who know you speak Dutch still switch to English, often without noticing. It is rarely about your Dutch being bad. It usually reflects social habit, politeness, or the other person's own eagerness to practice English.
Several things drive it. First, Dutch people are used to switching languages and may do it automatically when they hear any hint of non-native rhythm, even if they cannot consciously identify it. Second, many Dutch people enjoy speaking English and see it as friendly or helpful, not as a judgment. Third, in groups, one person may switch and everyone follows. Fourth, some people worry their own Dutch is not good enough for a serious conversation, which is ironic but real.
What you can do: keep replying in Dutch calmly and consistently. If they switch, answer in Dutch without commenting. If it persists, say once, lightly: "Laten we Nederlands praten, ik oefen graag" or "Ik vind het fijn om Nederlands te blijven praten." Most people will adjust. With close friends, you can be more direct: "Ik merk dat je vaak naar Engels schakelt. Is dat omdat je denkt dat ik het liever doe, of gewoon gewoonte?" That question usually reveals it is habit, not evaluation.
Be honest about the tradeoff: insisting on Dutch can feel awkward at first, and in fast group conversations you may miss nuance if you force it. Choose your battles. One-on-one conversations are easier to keep in Dutch than group settings. Also accept that some domains, like work jargon or jokes, may naturally drift to English.
Next steps: pick two or three people who are willing to stay in Dutch with you and make that explicit. In group settings, do not fight every switch; instead, re-establish Dutch when you speak next. If you want external feedback, record yourself and compare with native speakers, or ask a trusted Dutch friend for honest observations. Remember that reaching a level where people cannot tell you are non-native does not mean they will always behave as if you are a native speaker. Social switching is about comfort and habit, not just language ability. Your Dutch Fluency is clearly high; the remaining work is mostly social navigation, not grammar.