When the resource pack is not updated on the server side, the client requests the new version even if the resource pack exists in the cache However, the server returns an ETag on the GET request that should be used to compare the resource pack version.

It is suggested that the Minecraft client should store these ETags somewhere. For example, inside the downloads/{uuid} folder with the extension .etag, like this:

downloads/{uuid}:
40bd001563085fc35165329ea1ff5c5ecbdbbeef
40bd001563085fc35165329ea1ff5c5ecbdbbeef.etag

In the case where both the cached resource pack and the .etag file exist, the server resource pack should be requested with an If-None-Match header.