He could not channel chakra because he was dying. His organs were shutting down. The seal is not going to stay intact if he is dying.For the last time, he isn't heating in those panels, he simply has his organs damaged. It is only after Neji strikes him that he comments that he is unable to channel his chakra while we see in that same panel him losing his curse seal. Itisnt rocket science, ship Mate. Neji's strike was the reason kidomaru became incapable of using his seal.
With all the sound four, they die instantly (tayuya, jirobo, sakon) unlike kidomaru, who dies in a slow manner. So why do this? It was to show the effects gentle fist had on him. To say his seal stopped working when there is 0 evudmecr to support your claim while there being evidence that it was Neji's gentle fist strike that closed of his ability to access his chakra seal. It is literally written there in black and white, right after Neji hits him "I can't channel my chakra" while he loses his Curse mode v2.
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Hence the blood coming from his mouth.