This is actually a pretty typical thing in game engines. If you’re using variable replication like this, then you won’t get multiple updates for every time a variable changes. You’ll get some snapshot of it at the time the network API decided to sample the variable. Unreal is no exception here.
So if you add three items to your inventory in a frame, then the client should eventually get an update containing those three items (aside from weird edge cases like you remove those three items again and there’s a ton of packet loss that means that a client never gets the update containing those three items… I can’t remember if Unreal’s variable replication is “reliable”, but I’d assume that it isn’t), but you won’t get three separate RepNotifies, that’s not how variable replication works.
The only way to get multiple notifications like that is to use RPCs, but I wouldn’t recommend doing things that way. In general in multiplayer games it’s better to just use variable replication and live with the fact that there might be some weird edge cases where your client is temporarily out of sync, but that it will eventually get back into sync with the latest version.