It looks as if I was wrong. It is not a luaotfload problem, but of the generic fontloader. It doesn't look as if this will change, so you have the options
1. Add \automatichyphenmode=1
This will avoid the error at the cost that there will be no longer a possible line break point after the em dash (perhaps we will make this the default. It will also prevent line breaks after the hyphen e.g. for $p$-adic and if the hyphen is at the begin of the word, something that is good for some languages.)
2. Input the emdash directly: —. I wouldn't like this much, as I can type --- much faster than —, also --- is more visible, but it could be that it is your best option.
I suppose I'll go with option 1 for now, but it seems very unfortunate that luatex no longer behaves like every other TeX engine wrt this simple behavior. This is a large breaking change - it affects every existing document that doesn't explicitly set `\automatichyphenmode` or use Unicode em dashes. May I ask what recent work caused this?