Vertical slime block piston launchers have inconsistent behavior, sometimes they launch the player up and other times the player is pushed a single block with no vertical momentum. If the player is moving across the slime block(s) then the latter effect is far more likely to occur.
Also, when the player is launched the latter effect occurs for a couple of ticks before the former effect is applied.
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I have the same problem. It happens a lot more often on vertical bounce for me. And I do think it is a bug. Yes your position is different most of the time you use a slim block launcher, but that doesn't change the fact that a slime block should bounce you up if you stand on it. As long you are in range of the pre dot integer values of the slime block it should push you with slime block physics. And that worked fairly well in previous versions.
I dont believe this is a bug. It's because your position is not the same every time you use the slime block launcher. To fix the issue so it works 100% of the time, you need to sit in a minecart to fix your position