It's worth noting that the linked answer is primarily concerned with the Kwantung Army during the 1945 Soviet Manchurian Strategic Offensive Operation (sometimes known as Operation August Storm), but I think you're more interested in the earlier Soviet-Japanese border clashes like the Changkufeng Incident (Battle of Lake Khasan), and--most famously--the Nomonhan Incident (Battle of Khalkin Gol) that occurred throughout the 1930s as the Soviets and Japanese found themselves facing off across a massive and ill-defined border region between the Japanese puppet state in Manchuria and the Soviets in Siberia.

This question has several aspects that I'd like to approach. The first is that the Japanese were "easily" able to defeat the Chinese. While it is true that the Japanese won many victories against Chinese forces in the early phases of the Second Sino-Japanese Wars, many of these victories were far from easy. Just as an example, the Battle of Shanghai lasted for nearly three months, despite Japanese boasts that they could take Shanghai in less than a week. The Japanese also suffered nearly 60,000 casualties taking Shanghai, and while the Chinese lost more (and many of the lost Chinese forces were of the elite German trained divisions), it was hardly an easy conquest. The same could be said of the notable Japanese defeat at the Battle of Taierzhuang and the heavy casualties suffered in the campaign for Wuhan. While Japanese forces advanced, it was definitely not "easy" fighting, and Chinese forces put up strong resistance.

Yet, as you noted, the Japanese fared notably poorer against the Soviets than against the Chinese. I've discussed here how the Imperial Japanese Army (IJA) placed a greater emphasis on "will to victory" or spiritual power over material power, and this was a key aspect of how the Japanese intended to fight their battles in the anticipated war with the Soviet Union. The result of limited Japanese industrial capacity and the army's emphasis on spiritual superiority to overcome material inferiority was that--when compared to the Soviet Army---the IJA had substantial limitations in the quantity and quality of its artillery and armored forces, as well as logistical capacity. At Nomonhan, the artillery and armoured superiority of Soviet forces was telling, and that the Soviets were able to deploy and support such forces in a relatively remote section of the frontier came as a shock to the Japanese. As an example, the Japanese had around 800 trucks in all of Manchuria, while the Soviets used 4,200 to support their operations, enabling the Soviets to deploy and conduct operations much further from railheads than the Japanese thought logistically possible. Soviet superiority in heavy equipment, and importantly superiority in the logistical effort needed to support those weapons enabled the Soviets to inflict a decisive defeat on Japanese forces at Nomonhan. No Chinese force, be they Chiang's Central Army units, Nationalist aligned warlords, or Communist forces had the same level of equipment that the Soviets could bring to bear.

I hope this has helped answer your question, and please feel free to ask any follow ups.

You might find this earlier answer on a similar question by u/wotan_weevil to be helpful.