Youtube LiveChatMessages resource caching with ETag not work

My app every 5s requests live chat messages for YouTube live stream api and to prevent unnecessary data transfers I'm also utilizing ETags.

const auto listLiveMessagesRequest = yt::api::live::listLiveChatMessagesRequest(clientSecret).setParts({"snippet", "authorDetails"}).setFields("etag,items(id,etag,snippet(type,publishedAt,,displayMessage),authorDetails(channelId,displayName))").setLiveChatId(liveBroadcast->liveChatId).setMaxResults(200);mRefreshLiveChatFuture = std::async(yt::api::fetch, listLiveMessagesRequest.url, auth.accessToken, mLiveChatEtag);

https://developers.google.com/youtube/v3/live/docs/liveChatMessages/list

But I took note that my ETag caching has no effect on Live messages, since resource tag changes. As I suspect, its because that field constantly updated within resource:

pollingIntervalMillis

I compared two responses with different ETags and polling interval was the one changing.

Also, listing a live chat messages seems to have quite high quota cost of 5. Therefore eating 10k limit very easily!

Any other ways to enable resource caching? I notice of items[] that has their own ETags, but I wasn't able to figure out how request update only when items[] changed.

Resource caching with ETags to work as intended