I recently produced a sentence that made my friend learning Kílta ask which section of the grammar explained that use. Then I realized that not only was it not described in the grammar, I hadn't really thought about it explicitly.
Hakán ésamét kwan kwailo.
arm vaccine INST hurt.PFV
My arm hurts from the shot.
He wanted to know why the instrumental kwan was used here, when he would have gone for nós due to, on account of.
I have talked before about using a diary as a conlang tool. I am quite sure this sort of use of kwan started a while ago, but because the diary is handwritten, I can't easily search it to look for the first such use of kwan. Nonetheless, it was established early that kwan would indicate inanimate agents for passive verbs. The use of kwan in the example above is allied to that. It shows up in plenty of example sentences in the lexicon, chisanta kwan uttimo died from cancer, mata kwan atenko dissolved in the water, koska kwan haivo drown in shit, etc.
With a little thought it became clear that I was using kwan to indicate inanimate or indirect agents in patient intransitives (also known as "unaccusative verbs," an excessively cute and confusing bit of terminology). These are intransitive verbs where the grammatical subject doesn't have much agency in the situation, die, fall, be sick, happen, hurt, etc.
The detransitive suffix -is-o generally results in verbs with more patient-like subjects, so it, too, can take kwan in this sense,
Chátis në mëtaula kwan kwitiso.
window TOP storm INST break.DETR.PFV
The window broke in (due to) the storm.
So here was a bit of Kílta grammar that was (probably) created in the diary, got used all over the examples, but hadn't been expressed explicitly until I got asked about what was going on. This is normal in the diary process. Certain use patterns develop because they seem right at the time, and over time take on semantics that can be hard to explain at first. In this case, I'm lucky enough to have someone ask me what I was up to with kwan. And now it's explained in the grammar.