I think the HPF-part is missing in the graphics for the LPF + HPF element filters.

Why do you think that? It is not the case. A LPF is a Low Pass Filter - which literally, allows low frequencies to pass through and blocks high frequencies. A HPF is the opposite, it allows high frequencies to pass and removes low frequencies. Here's what's going on:

The LPF+HPF Filter Types feature the LPF in series with the HPF, the graphic (which I'm going to guess is letting your eyes confuse you) on the left is the LPF, the graphic on the right lets you dial in the HPF. (Had you let your ears draw the conclusion it would have told you an entirely different story.) And with sound we should always believe your ears. 🙂

Here's how you can HEAR this dual filter.
Let's recall a sound with strong harmonic content.
Recall the Performance "Rap Lead 1" a good example because it is not only a single Part, but it's a single Element which will let us hear it by itself, and has plenty of harmonics to filter.
It's in the Synth Lead: CATEGORY.

It also happens to use the LPF12+HPF12 Filter Type.
As mentioned, the parameters and graphic on your left control the LPF12 dB/oct filter. You have Cutoff, Cutoff to Velocity, Resonance, and Resonance to Velocity. On the right side you have the HPF12 dB/oct and the following parameters Cutoff scale balance above and below the C3 centerpoint, HPF Cutoff Frequency, and HPF Cutoff Key... Finally an overall GAIN after the filter.

When you first recall the Part it appears as below:

Play chromatically across the keyboard to get a feel for what is allowed to pass in this condition. Then try changing the HPF Cutoff Key to -200% notice how middle C (C3) sounds normal, but by the time you play chromatically down an octave a lot of lows are gone, and by the time your down a second octave the sound is almost completely filtered. HPF allow highs to pass and block lows... thins the sound.

Hope that helps.