In Release 5.18.1 introduced MetaData Map with signature Map<String, Object>
You missed adjusting CustomerService CustomerCreateRequest.withMetadata(Map<String, String> metadata) - this is still using the old signature - see https://github.com/gocardless/gocardless-pro-java/blob/master/src/main/java/com/gocardless/services/CustomerService.java#L201
In Release 5.18.1 introduced MetaData Map with signature
Map<String, Object>You missed adjusting CustomerService
CustomerCreateRequest.withMetadata(Map<String, String> metadata)- this is still using the old signature - see https://github.com/gocardless/gocardless-pro-java/blob/master/src/main/java/com/gocardless/services/CustomerService.java#L201