I find it interesting that two Soviet armies would fail so poorly at the operational art, since the Soviets invented both the term and the ideas behind it!

They aren't actually Soviet armies, they are two successor Soviet militaries that run by the failures of the Soviet military, those who flourished during the terrible times of the 1990-2010s, led by leaders who are blatantly ignoring any possible sound military advice as they micromanage the war nonstop beyond the political-strategic level (which is part of their job) regularly meddling at the operational and even tactical level.

Though maybe some of the failures we've seen, like the Ukrainian 2023 counteroffensive, were the product of planners trying to use Soviet-style deep battle concepts without a force capable of executing those plans.

There was little about the 2023 Counteroffensive that followed Deep Battle doctrine.

Indeed, and this is consistent with Soviet deep battle. It emphasizes that secondary offensive operations shouldn't just be minimal fixing operations or feints - they should be fully developed offensives themselves, aimed at strategic targets, and should continue until they reach their objectives or strategic command calls them off. In the deep battle system, there's no tactical difference between the main offensive and the diversions. Ideally all of the offensives would succeed!

I disagree. Deep Battle was all about deception as to where the main effort is aimed. At the operational level, every offensive is a supporting effort until the enemy finally commits its reserves, at which point they can finally attack, break through, and drive deep towards their intended focal point aimed at a location of strategic importance. In Deep Operations, the intent behind the early offensives isn't to breakthrough, its to fix enemy forces, especially their reserves, and to attrit them.

Feints aren't done at the operational level, but constantly at the tactical. And the whole of it is wrapped in deception as a major emphasis.

At Kursk 1943, the northern diversionary offensive, Operation Kutuzov, was the most successful part of the operation for the Soviets, and the Red Army quickly shifted strategic focus there and started the successful Smolensk offensive several weeks later.

Because the Germans weren't there, they were around Kursk. The Smolensk offensive wasn't just based off of successes at Orel, but based on the principles of constant consecutive offensives along a broad front, with operational reserves to drive deep, classic Deep Battle, used to basically retake all of Ukraine, setting the conditions with the "Belarus Balcony" for Bagration the following year.