https://projects.propublica.org/represent/members/S000033/vo...
An absent vote is the same thing as a nay vote in the Senate. To pass this amendment it requires 60 affirmative votes, of which there were 59.
It's "yea" for requiring warrants, and "nay" for warrantless surveillance (which is somehow the law of the land now? When did I miss this??)
I suspect Bernie's abstention is because he thinks reauthorization is equally unacceptable with or without the condition.
Procedurally, he could vote yes on the amendment and still vote on the bill, and there's an argument that not doing so is making the perfect the enemy of the good (or at least the barely decent the enemy of the mildly less oppressive of two oppressive options), but it's kind of hard for me to get mad at Sanders for not collaborating in putting lipstick on this particular totalitarian pig of law. One can maybe rationalize the USA Patriot Act in the context of it's time, it's much harder to rationalize the USA Freedom Act’s deliberate after-the-fact authorization of much of the illegal blanket surveillance that went beyond what the original Patriot Act authorized, and it's far harder to rationalize the current reauthorization of the combination, with or without minor tweaks.
Assuming Bernie would have voted YEA on this, we're all wondering why he didn't vote at all.
In this context, it wasn't as critical of a vote. I wouldn't be surprised if Schumer allowed the vote to happen today while Sanders was absent to drum up some extra support for Biden.
The real voting is often done at the party whip level. If McConnell knew Bernie was going to show up and vote, it's possible that another Republican would have voted nay.