It seemed much more natural to say "Those/the sentences are beautiful". Nice just felt weird. Beautiful was marked wrong, which I thought was weird, as it seemed like a good translation. I know technically "krásné" is beautiful, but I agree "nice sentences" sounds odd.
It is a fixed rule, but it is not formulated the way you mention.
The rule says that an agreeing modifier is placed before the head (the noun), see "premodifier" in the link to Wikipedia. An agreeing modifier agrees with the head in the case and number and it is typically an adjective. Such a modifier is placed as an attribute of that noun.
What we have here is an adjective in a predicate connected to the subject using a connection verb (copula). Rules about modifiers are irrelevant for predicates.
These rules work pretty much the same for Czech and for English. They differ for Czech non-agreeing modifiers.
Attribute (adjective modifying a noun) - the adjective comes first (same in Czech, English, German etc., but different from Spanish, French, Polish etc.):
- dobrý muž - a good man
- mladá žena - a young woman
Predicate (X is Y) - again, same in Czech and English and other languages:
- Muž je dobrý. - The man is good.
- Žena je mladá. - The woman is young.