[MEI encoding] Syllable should not have both `precedes` and `follows` attributes

In the latest version of rodan.staging, syllables sometimes get encoded with both a `precedes` and a `follows` attributes. This shouldn't happen.

This file contains an example, ID: m-abb9b807-fc06-4a94-8cf8-b0770b21e81d.
[165v.mei.zip](https://github.com/DDMAL/Rodan/files/13545575/165v.mei.zip)

It's also weird that all three syllables involved are on the same staff, which means they don't have to be toggled in the first place. I'm noticing, however, that the syllable that's toggled twice is between two custodes. Is that what's making it toggled?

@yinanazhou what should happen if a custos is in the middle of a syllable? In Einsie, a custos only appears before a staff break, but sometimes if the IC identifies stuff wrong a custos will end up in the middle of a staff in a straight-outta-OMR file. Is a toggle the correct thing to do in that case? Or should the custos be inserted into the syllable?