Hey Victor.
You appear to be able to put the spine in extension, you just need to do it harder, and hold it. As the set progresses you tend to not squeeze up as hard, and the back is never fully "set". You should squeeze up so hard that the bar wants to come off the floor. Harder than you think you need to. Every time.
I can't tell if you're taking a big breath and bracing the abs to keep the back tight. That may also be a contributor if you're not.
Once you get consistent and getting the back squoze (past tense of squeeze) into extension, it may be time to trying out a belt for the deadlift.
Good luck.