Its creator probably didn't use the right setting. Please report the game so that staff can check.

How is what exactly possible?

Was the game marked as adult or was it not? Did the message about adult content appear or not? This was not clear, especially since you act like developers could not forget this (it happens. Itch's interface could be a lot better) or even do that intentionally (usually scammers).

Or is it, that you do not realize that tags themselves are not important for this? You can have an adult game without any tags at all. The tags are only important for browse, as any of the "adult" tags and your setting  will be ignored.

But using such tags will not put your game in the adult category. You need to click a checkbox in the metadata. A rather hidden checkbox, I might add.

Audience & Content

• Has sensitive content — This project is not suited for minors or the workplace

• Show content warning — Show sensitive content warning before accessing game

But thinking about this, why are there two checkboxes? What is the difference between selecting only the upper vs selecting both (the lower cannot be selected unless the upper was selected).

Is not the meaning of the upper verbatim nsfw? Why is it optional to click to show the sensitive content warning?

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Or was it said somewhere that selecting any "adult" tag does indeed sets your meta data? That might have been a server hickup.