Word order or emphasis. The verb "bejön" is intact with its preverb, so it demands the emphasis. Yet, the word "konyhába", describing a location, is placed in front of the verb, between the subject and the verb, as though it wants to grab all the emphasis. One of them has to give:

"A pincér is a konyhába jön be."
"A pincér is bejön a konyhába."

Curiously, this one could also work:

"A konyhába bejön a pincér is."

Also:

"A konyhába a pincér is bejön."

Just do not place anything else between the subject and the verb and leave the verb intact. That will cause a conflict.