AC does not clean up the joints between slabs on floor plan (longstanding wish) so you are not doing anything wrong. The usual solutions/work arounds are to
• Turn the slab layer off for floor plans (recommended)
• Draw a white line or white fill over the joint
• Use GOs to override the uncut pen for those slabs (have not tried it)
AC floor plan views are not a projection.
You have found one of the annoying little items in AC for which all users have their workarounds.
AIA NCARBAC26 US/INT -> AC08
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Learning: VW, RVT
Teaching & Using: AC, Rhino, ACAD, RVT, SU, AI, PS, InD, TM, C4D, ATL