I'm sure there is a rather complex set of rules with a lot of historical precedent about how dictionaries split syllables, but the rule is to follow the dictionary. You are right, you wouldn't pronounce it "nat-u...", you'd pronounce it "na-tu...," but this is why musicians are always reading ahead in the score. If you aren't reading ahead, you'll get notes wrong, rhythms wrong, miss dynamics, get behind the beat... and you'll also mispronounce lyrics.
Of the dictionary rules I know, one is that prefixes and suffixes are always split off from root words as their own complete unit, so you get "in-tens-i-fy" instead of something like "i-ntens-if-y," you always split between double consonants, so "nat-u-ral-ly" actually obeys the double consonant rule and the splitting of a suffix rule, and then there are some rules about how you divide up chunks in words that aren't so clearly suffix or prefix.
Partly I think the rules are meant to help chunk words into parts so that we pronounce the vowels correctly, even if we aren't reading ahead, since in English the way we pronounce our vowels is variable, but often follows a pattern based on the consonants around the vowel. If "pattern" was chunked as pa-ttern, you might read the first syllable as "pa" rhyming with "father," but if you chunk it as "pat-tern" then you intuitively read it as "pat" rhymes with "hat." It's a rule for a reason, even if we don't always know the exact reason. (My neighbor edits dictionaries for a living, so she might be able to tell you why). 🙂.
If you want a good guide to this sort of stuff, I highly recommend "Behind Bars" by Elaine Gould. She was an editor at one of the big music publishing companies for decades, and had sets of handouts for the junior editors she supervised to keep everyone consistent, which got longer and longer until they turned into a doorstop of a style guide book. It explains the current way major publishers do everything from writing extended techniques properly for timpani, to when to write a rhythm as two tied notes instead of a longer dotted note, to what order to put different combinations of instrumental parts top to bottom on your score. It's not that people can't read the music if you do things other ways, but some ways are clearer on the page for one reason or another, (she often explains why), and if we all do things the same way, people get used to the pattern and don't have to think about it. Spelling rules don't always make the most sense, but if we all agree to spell everything the same way we get used to it and don't have to stop and puzzle over what someone was trying to say. And when there are rules that the major music publishers have agreed to, if you ignore them without a good reason, your score looks like a social media post about conspiracy theories full of WEIRD all-Caps, LIKE YOUR CRA-zy relative wrote it.
And yes, each choral part always gets its own set of complete dynamics, even if it's homophonic. Two or more parts don't share dynamics. (They can share lyrics, though). That's another rule. Order "Behind Bars." You'll love it! 🙂