Spring Cloud Gateway in Spring BFF - Rewrite Response function not executing

Question

I don't understand why this filter, the rewrite function is not executing. The re-write function does not transform anything, but should just log out a simple message.

override fun apply(config: Config): GatewayFilter = GatewayFilter { exchange, chain ->

        // only modify /user-info endpoint responses
        if (exchange.request.path.value() != "${serverProperties.resourceServerPrefix}/user-info") {
            logger.debug("request did not match userinfo endpoint")
            chain.filter(exchange)
        } else {
            chain.filter(exchange).then(
                exchange.session.flatMap { session ->
                    logger.debug("request did match userinfo endpoint")
                    val sessionId = session.id
                    val sessionKey = "${sessionProperties.redis?.sessionNamespace}:sessions:$sessionId"

                    redisTemplate.opsForHash<String, Any>()
                        .get(sessionKey, "creationTime")
                        .doOnNext { creationTime ->
                            logger.debug("3. Found creation time in Redis: {}", creationTime)
                        }
                        .flatMap { creationTime ->

                            val created = Instant.ofEpochMilli(creationTime.toString().toLong())
                            logger.debug("4. Converted to Instant: {}", created)

                            // Define the RewriteFunction separately
                            val rewriteFunction = RewriteFunction<String, String> { _, originalResponse ->
                                logger.debug("5. Original response as string: {}", originalResponse)
                                Mono.just(originalResponse) // Return the original response as is
                            }


                            val modifyConfig = ModifyResponseBodyGatewayFilterFactory.Config().apply {
                                logger.debug("Setting up rewrite function")
                                setInClass(String::class.java)
                                setOutClass(String::class.java)
                                setRewriteFunction(String::class.java, String::class.java, rewriteFunction)
                            }

                            val modifyFilter = modifyResponseBodyFilterFactory.apply(modifyConfig)

                            // Apply modification filter first, then the chain
                            modifyFilter.filter(exchange, chain)
                                .doOnSubscribe { logger.debug("Subscribed to response modification filter") }
                                .doOnSuccess { logger.debug("Modified response completed") }
                                .doOnError { error -> logger.error("Modification failed", error) }
                        }
                        .onErrorResume { error ->
                            // log the Redis error but allow the chain to continue
                            logger.error("Failed to get session timing from Redis", error)
                            Mono.empty()
                        }
                }
            )
        }
    }

The only things that get logged are:

2025-01-15T20:13:33.074Z DEBUG 32653 --- [BFFApplication] [ctor-http-nio-4] c.v.b.r.s.SessionInfoResponseFilter      : request did match userinfo endpoint
2025-01-15T20:13:33.091Z DEBUG 32653 --- [BFFApplication] [xecutorLoop-3-4] c.v.b.r.s.SessionInfoResponseFilter      : 3. Found creation time in Redis: 1736971305338
2025-01-15T20:13:33.091Z DEBUG 32653 --- [BFFApplication] [xecutorLoop-3-4] c.v.b.r.s.SessionInfoResponseFilter      : 4. Converted to Instant: 2025-01-15T20:01:45.338Z
2025-01-15T20:13:33.092Z DEBUG 32653 --- [BFFApplication] [xecutorLoop-3-4] c.v.b.r.s.SessionInfoResponseFilter      : Setting up rewrite function
2025-01-15T20:13:33.096Z DEBUG 32653 --- [BFFApplication] [xecutorLoop-3-4] c.v.b.r.s.SessionInfoResponseFilter      : Subscribed to response modification filter
2025-01-15T20:13:33.096Z DEBUG 32653 --- [BFFApplication] [xecutorLoop-3-4] c.v.b.r.s.SessionInfoResponseFilter      : Modified response completed

What can I try next? I can't see where I've gone wrong.