Just in case anyone else comes across this thread, there’s already been a lot of interesting discussion of this issue, one forum category over. I missed it at first. “lg 5k” doesn’t return search results, but “5k display” does.
LG UltraFine 5K Frequent Connection Drop-off and Resets (#6265) · Issues · drm / intel · GitLab is the bug that I’ve been encountering. That thread reports the problem across both Windows and Linux, including Ubuntu on some Lenovo laptop. Doesn’t sound like it’s limited only to Framework laptops.
Lastly, if I set “both” LG “displays” to 2560x2880, then the monitor renders both of them side by side. Between KDE and Xorg, they aren’t presented as being a single display on the software side.
For now, I’ll keep tolerating it as-is until I get the right set of kernel updates and somebody forces me to use Wayland. Running it as a 4K monitor isn’t that bad, either, and it’s a smoother workflow with the fractional scaling looking alright at 125% (same as the built-in display).