Hello sukkamehu,
Welcome to Microsoft Q&A forum.
For me, I'd not trust 1800 as the correct monitor peak brightness. While it is true that the maximum brightness can go higher than what stated by the manufacturer, it usually won't go that high. In this case, I believe that this happens because tone mapping is delaying the disappearance.
That being said, the 1800 value won't harm the monitor in anyway as I know. So if you like the look of it, it is totally fine to keep it that way and use HDR. I hope this clarifies.
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