At https://github.com/graphql-java/graphql-java/blob/v18.1/src/main/java/graphql/schema/idl/SchemaPrinter.java#L678, the SchemaPrinter calls out.format on a string with directive, but doesn't supply any formatting arguments. If the directive string contains anything that resembles a format specifier, then the call blows up.
For example, it breaks when there's a constraint requiring a value ending with percentage sign: @constraint(pattern : ".+%")
See related smallrye/smallrye-graphql#1492