I have a PNG essentially borrowed from your long thin line in the Scrivenings view.
I inserted code for that in Compile/Formatting/Layout/Prefix at the Text 2+ level:
and that works super all the way up to chapter 53. For instance:
But it’s gone in chapters 54 onward. That REALLY looks like a bug to me.
ADDENDUM: when I switch that code to Suffix, not Prefix, it fails after 24 chapters.
I sent the project to Jeff at L&L support.
The problem is actually intermittent; the separator fails at chapter 25 (maybe sooner) either way, prefix or suffix. I’m not sure if the failures are identical both ways, but they may be.
RESOLUTION: some documents had been (somehow) marked to ignore prefix and suffix. The scorched earth solution was to open the .scrivx file inside the package in a plain text editor (Mac’s TextEdit did it for me) and replace this string:
with nothing. There were 109 substitutions as a result, some of them in the Research and Template folders. I then went to Compile/Text Adjustments, selected the setting gear and gave that attribute BACK to one document that actually needed it, an Intro page before the first chapter.