Just found these messages in my logs:

17:04:43,731 (main) DEBUG FileLookup:57 - Unable to find file schema/infinispan-config-5.1.xsd in classpath; searching for this file on the filesystem instead.
17:04:43,731 (main) DEBUG InfinispanConfiguration:389 - Could not find schema on path schema/infinispan-config-5.1.xsd, resolving infinispan.config.schema to http://www.infinispan.org/schemas/infinispan-config-5.1.xsd
17:04:43,911 (main) DEBUG InfinispanConfiguration:396 - Using schema http://www.infinispan.org/schemas/infinispan-config-5.1.xsd

Indeed the schema is included in the jar, but is in the root of the jar, while it's searched for in schema/infinispan-config-5.1.xsd