Yeah, I feel you. I had very similar experience. After the party of four fifth level characters cleared an encounter that was about CR 14,* I realised that the CR is a joke and "deadly" means nothing. There was also a few dragons (that were boosted) which I meant to be really tough fights but ended up being cakewalks. So I have just been throwing all sort of crazy stuff on them completely disregarding the guidelines. But I am just eyeballing stuff as there is no guidelines for whether quadruple or sextuple deadly actually is too much.
(* it was not meant to be a combat encounter, but the PCs started a fight against people with overwhelming power and numbers. I though it was suicide. It was not.)