Just to help us (me) understand, what specifically is not working? What do you get instead of what you expect?

(And your computer OS and version is?)

EDIT: Okay , I see from your other past you are on a Mac.

Hi…

Yes, all relevant info should be in the footer on my post.

What's *not* happening is, when I change the bar line at the end of one system, say to a double-bar, the left bar line of the next measure (on the next system) does not change like it’s supposed to.

Richard can you  post an example?

See attached. (And yes, I did check these settings and click "OK" first. I just re-opened the dialog so you can see my settings.)

Curiously, this IS working on NEW docs created in v27.2, but is not working in docs or doc templates created in v26.

Not sure if this is related to my issue, but for ages (predates F27 by a lot), if I use dashed barlines and want them as my default, checking Default Style is Previous Measure's Right Barline doesn't cause new measures to have a dashed barline. What saves me is a .lua script that with one click, makes every existing measure in a work have dashed barlines. And yeah, I could certainly do as I used to: select all then right click and change the barlines. But the .lua script saves me a click or two. Anyway, the way at least it would appear to work in Document Options: Barlines doesn't.