STR.

1. goto https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=243229907&repo=mozilla-central&lineNumber=10308
2. search flac-68 and select hightlight all
3. scroll page down or up to the place where you can't see any hightlight texts
4. scroll back to the place where highlight texts exist

Expected.
5. still see text highlight

Actual.
5. no text hightlight

I reproduced this issue on Firefox Release 66.0.3 on Unbuntu 18.04.1.

I can also reproduce this issue on Mac.

Ough, this looks like a weird interaction with the logviewer used here. I'm betting that this issue doesn't appear when you're viewing the raw logs. Looks like it's using a virtual-viewport React component, which burps fresh data into it on scroll.
I don't think this is something I can fix, unless I turn on modal highlighting by default.