Dear Khun Thanachok,
Thank you for your question.
As discussed with Khun Nusara-P49, the interior designer allow to have control joint at the connecting area. You can fill in this small gap with sealant or backing rod and some inlay strip.
In terms of aesthetics, could you please ask the contractor to submit the shop drawing to interior designer for approval before construction including the sealant and relevant material and method.