I Don't Think You Can Explain This (Terraria Housing Mechanics)

[FAdadadada]

@Flaef

Is it because the gaps in between the diagonal platforms are not large enough to meet the invalidation requirement for a continuous unwalled space?

Correct

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@Flaef

However, if I remember correctly, there is an alternative wall requirement stating a specific number of walls which can be missing in a house, connected or not, before the house is invalidated for the lack of them.

There is no such requirement. Houses can be valid with no walls whatsoever.