Currently the default is Tank -> Healer -> DPS for how the unit frames are arranged when using By Role layout.
In classic era you usually have one priest in each group for prayer of healing, or one shaman in each group for totems.
I think it'd be cool if I could choose them to always be ordered last. Instead of sometimes being first player in group without tanks, or if the tanks are in the group they are 2-3rd player in group.
Preferably being able to do this without having to create a custom layout that can be annoying to share (people never press include custom layouts when importing profile :).

Try out 2.9.7-beta version.
Setting to change roles order at "Layouts / General"
This setting is only applied to grid2 built-in layouts (custom layouts are not affected).

I had a group today with 3x tanks, 1x dps, 1x healer that had incorrect order.
T T D T H - tanks & dps was seemingly sorting alphabetically. This was with the layout By Group & Role w/Pets and role order Tank>Damager>Healer>None

https://imgur.com/Bq560FD
https://www.twitch.tv/videos/2586436970?t=0h50m27s

Maybe the issue is related to theme switch. Could you show me the layout settings for the "default" theme ?

I set the same roles order in the default when I did it for the 40man theme

https://imgur.com/E5010gX

profile: https://pastebin.com/dqatqRwM

i can see nothing wrong with your profile or the code added.
There is only one line of code added to change the the sort order template, and if the line is not executed the game uses the standard sort order: tank, healer, dps, none. So a order different than "T,H,D,N" or "T,D,H,N" should not be possible.
The issue should not be related to the new code added.
Report back if the issue happens again, next time you may check out if a reload ui fixes the order or not.

I have not been able to replicate it again, even with same players.

Same issue happened again, with hunter being between tank roles.
https://i.imgur.com/cZkjcgr.mp4