It is worth separating two things that are easily run together: that the arrangement of the Guru Granth Sahib is deliberate, and that its logic is known.
The first is beyond doubt. The volume is organised to a scheme applied consistently across thirty-one sections and several thousand compositions, with the ordering of authors, of forms and of lengths holding throughout. Nothing about that is accidental, and the regularity is tight enough that departures from it are used as evidence.
The second does not follow. The scheme within a section is legible because it is systematic and can be read off the pages. The order of the sections themselves is legible only as a fact. The volume gives no preface explaining it, the Gurus left no statement about it, and the early manuscripts do not vary in a way that would expose the reasoning.
Readers meet plenty of confident explanations, and they are inferences. Some are attractive: that the sequence opens grave and closes at dawn, that it moves through moods, that it reflects a musical relationship between measures. Each has been argued and none commands assent.
Recording a gap of this kind, rather than filling it with the most plausible available account, is the harder discipline and the more useful one. Someone may yet close it with manuscript evidence, and a reference work that has already printed an answer makes that work harder rather than easier.