RIP version 1 route advertisements do not include a field for the netmask. This means receiving routers have to gess the netmask based on whatever information they have available. That includes the "natural" netmask of that classful network. That also includes the receiving routers own configuration.
This means that variable length subnets and discontiguous classful network have problems with RIP version 1. The lack of the netmask field in the route advertisement is a real problem.