More info:
I think I know what happened. MUBI’s subtitles are in XML format. Even when they’re “converted” to SRT by StreamFab and the tools it uses, they’re literally just the raw XML renamed .srt. MKVToolNix doesn’t recognize them as valid subtitles when I try to add them to a mux.
Pretty sure this is why it’s erroring out.
EDIT: Subtitle Edit can read and convert them to “proper” SRT.