If that tiny tweak solved the problem, great -- you don't really need any more advice. But, this being TB, I feel compelled to offer some anyway.
It seemed to me in your video that you're getting the slap sound mainly when you play the D (and probably G) string, but not the A or E. I think that's because as the distance between your thumb-anchor and plucking finger increases, the angle at which your finger meets the string changes so that you are pressing the string more vertically (down toward the bass body) and less horizontally (toward the next string). You're using a "moveable anchor," which is great, but ideally this technique would mean that your plucking position and finger angle remain exactly the same across all strings. You're already anchoring on the pickup to pluck E, and on the anchoring on the E to play A, so maybe try anchoring on the A to play D and on the D to play A.
Long ago I experimented with both the moveable anchor and the alternative "floating thumb" technique, and came to the conclusion that I preferred the latter. The basic idea is the same, except your thumb lies and slides horizontally across the strings instead of anchoring anywhere, and you move your entire hand and arm when crossing strings with the thumb in tow. Just for giggles you might want to give it a try and see if you like it:
The main reason I personally prefer this is that I just find it easier to "float" the thumb this way than having it jump from the string to string in order to keep my plucking motion identical across strings. But the bonus benefit I discovered is that this technique makes it difficult to "dig in" and pluck harder than necessary. My plucking fingers just brush across the tops of the strings rather than really "plucking" them -- and I really like playing with a very gentle touch (and super-low action).
None of this is necessary for you, of course, but you seemed curious to learn so I thought you might find it helpful.
