Automatic bracketing is set in Layout Options on a per-layout basis. Always go there first before adding manual bracketing changes, because manual bracketing changes apply left-to-right and based on the staves that were visible at the time they were set; they can get a bit stuck if you later add/remove staves etc.
Here is a version of your project with a (fairly hastily, I might add) alternative with section players, who then divide in the 2nd movement. Divisi brings automatic sub-brackets.
Whether or not you want this style of divisi depends on your intention: you said this “isn’t really divisi”, if you mean you’ll have separate soloists who only single the solo part, then maybe divisi isn’t appropriate. If the solo voices come from within the choir, then divisi probably makes sense.
I’ve not double-checked all the details, so if you wanted to use this project, I’d recommend proof-reading it against your own.
You might have had issues with divisi because your Tenor was set up as a single player, not section. Single-type players can’t do divisi. I made yours a section player by adding a section player (but not giving it its own instrument), then dragging the existing Tenor instrument to the empty-handed section player.
When the voices all sing the same part, you can restore a unison and go back to single staves whenever you like.
As to why you weren’t getting sub-brackets initially: the Alto/Tenor instruments are different. The alto solo is “contralto”, so that won’t sub-bracket with “alto” automatically. Ditto, I think the tenors were two different sorts of tenors – changing the “T” tenor back to a tenor singer brings sub-brackets in your project. Take care when adding instruments that you’re adding the correct ones.
On a side note: be careful about your dynamics and hiding rests. There are missing rests in your second song, because loads of notes/rests have the “ends voice” property set. Elsewhere, you have gradual dynamics with mismatching durations, or duplicate gradual dynamics on the same staves. Maybe this is something that you’re planning to do in a tidy-up, but thought I’d mention it.
I also removed the option to use the player name for the rehearsal piano in staff labels: using the instrument name allows Rehearsal / Piano to appear on 2 lines, as you set for the instrument name.
ThreeWelshChildrensSongs w brackets_LH.dorico (2.2 MB)