Neither of your examples seems correct.

The noun following “ce genre de” should be singular (as it is in English) because “genre” is singular as well, and there should be no confusion in conjugating the subsequent verb.

Your examples should read:

Parfois, ce genre de coïncidence a tendance à s’enchaîner.

and

Quel dommage que ce genre de soirée soit si rare.

However, you can also say “ces genres de…” in which case your examples would be

Parfois, ces genres de coïncidences ont tendance à s’enchaîner.

and

Quel dommage que ces genres de soirées soient si rares.

I think it’s not realistic to think about formal “rules” in this(es) kind(s) of case(s). Rules are quite flexible or almost inexistent in practice.

However, the tendency is probably a plural agreement (with the gender of the entities) if you refer to entities of the same kind as the present one(s):

Ce(s) genre(s) de métaux sont maléables.

Ce(s) genre(s) de soirées sont en général très courtes.

In this case, you’re free to consider only one precise kind or more than one.

And, even more likely, the agreement will be done with “genre” itself, if “genre” really refers to the kind(s) itself/themselves, not to a group/groups of entities which share a similarity.

Ce genre de libellule(s) n’a pas été répertorié.

Ces genres de libellule(s) n’ont pas été répertoriés.

In this case the number for the entities will likely depend on how many you picture in your head.