reactor.netty error when depending on spring-boot-starter-webflux - spring-boot

Question

I have a java project based on Spring framework, which is based on gradle build tool.
The gradle configures a parent project Par and sub-project Child.
The Par project has the build.gradle setting spring framework boot version as 2.2.6:
plugins {
id 'org.springframework.boot' version '2.2.6.RELEASE'
...
}
The Child project has the build.gradle as below:
ext.versions = [
spring: '5.2.9.RELEASE',
spring_boot: '2.2.10.RELEASE',
slf4j: '1.7.30',
grpc_helpers_java: '^19.0'
]
I tried to use WebClient of spring framework, so I added spring-boot-starter-webflux dependency to the build.gradle file of the child project as below:
dependencies {
...
compile group: 'org.springframework.boot', name: 'spring-boot-starter-webflux', version: versions.spring_boot
...
}
I don't have other code change yet.
However, although the build passed, the bootRun from the Par project failed with the following error:
An attempt was made to call a method that does not exist. The attempt was made from the following location:
org.springframework.boot.web.embedded.netty.NettyReactiveWebServerFactory.createHttpServer(NettyReactiveWebServerFactory.java:163)
The following method did not exist:
'reactor.netty.transport.Transport reactor.netty.http.server.HttpServer.runOn(reactor.netty.resources.LoopResources)'
The method's class, reactor.netty.http.server.HttpServer, is available from the following locations:
jar:file:/C:/Users/XXX/.gradle/caches/modules-2/files-2.1/io.projectreactor.netty/reactor-netty/0.9.12.RELEASE/41022546d07f1499fb9d8617bba4a1a89d3549db/reactor-netty-0.9.12.RELEASE.jar!/reactor/netty/http/server/HttpServer.class
The class hierarchy was loaded from the following locations:
reactor.netty.http.server.HttpServer: file:/C:/Users/XXX/.gradle/caches/modules-2/files-2.1/io.projectreactor.netty/reactor-netty/0.9.12.RELEASE/41022546d07f1499fb9d8617bba4a1a89d3549db/reactor-netty-0.9.12.RELEASE.jar
Action:
Correct the classpath of your application so that it contains a single, compatible version of reactor.netty.http.server.HttpServer
I think the transitive dependency reactor.netty has a version of 0.9.12 is correct based on the spring-boot-starter-webflux version of 2.2.10. So I do not understand why the bootRun is trying to look for reactor.netty.http.server.HttpServer.runOn method which doesn't exist for reactor.netty version 0.9.12
Thank you very much.