I encountered some behaviour that doesn't seem right to me:

When I press a key A, then press the sustenuto pedal, then release the key: The sound of the key continues, it isn't damped.

This is expected behaviour.

If I now press another key (while the pedal is still held down), release the sustenuto pedal (sound of old key stops), press the sustenuto pedal, and release the key:
The sound of the new key isn't damped and continues as well, and this is as well expected.

However, the first key's damper is still up: When I now press and release the old key, the sound still continues.

I was also able to reproduce this result with a longer chain of different keys. Wether this occurs seems to also depend on how long it takes me to press and release the pedal, and the keys. I can however assure you that the pedal wasn't partly pressed when I released it. I also didn't touch any other pedal at all. I'm using pianoteq 7.5.2 on Windows (64 bit). Is this a bug?

To be clear, if you press and release the pedal quickly, the "problem" occurs, whereas if you let the pedal in its rest position for a somewhat long period of time it doesn't? If so, it's possible that PTQ is simulating the behavior of a physical acoustic back action. I never actually tried this on an acoustic grand, but it sounds mechanically plausible: it will take a bit of a time for the back action (damper) to lower down, and hence if you release and repress the pedal too quickly it is possible that the back action could be re-engaged into the sostenuto rod. If it's a feature, I'd expect it to be configurable.

Regarding the "ignore the sustain pedal", its description appears to force the sostenuto to exactly ignore the condition I mentioned below, even thought it mentions it in the context of keeping the dampers high with the sustain pedal, rather than the simple fact that lowering them takes time.

What is unclear about the description?

I am not able to reproduce the issue using the steps you describe, and using my MIDI keyboard and Pianoteq. I just thought that the "ignore sustain pedal" option might help.

In your fifth paragraph you write: "However, the first key's damper is still up: When I now press and release the old key, the sound still continues."

That is not true in my case, with my equipment. The "old key" (the first of the two keys/notes depressed) resonates as though its damper is disengaged only if I have not released the sostenuto pedal since it was depressed after the first note's key was struck. This last statement is true, in my case, whether the "ignore sustain pedal" option is engaged (checkmarked) or not.

So perhaps the issue has some relation to one's hardware and its MIDI implementation: how one's keyboard translates the movements of the pedals attached to it into MIDI values and transmits the MIDI information, or whether one is using a pedal that acts as a continuous-controller rather than a simple on-off switch to control Pianoteq's sostenuto pedal.

You might check the MIDI pane of Pianoteq's Options window to see which MIDI data your keyboard and associated hardware is transmitting, and which MIDI controller(s) are mapped to trigger the Pianoteq Sostenuto pedal.

Your description of the issue is not entirely clear. But  try the following:

Right-click the sostenuto pedal, and enable (checkmark) the "ignore the sustain pedal" option.

Does the issue persist?

What is unclear about the description? I can improve, if needed. I wrote down every step.

"Ignore the sustain pedal" does indeed stop the behaviour. It's just unexpected, as I don't do anything with the sustain pedal.

Right-click the sostenuto pedal, and enable (checkmark) the "ignore the sustain pedal" option.

Does the issue persist?

When I press a key A, then press the sustenuto pedal, then release the key: The sound of the key continues, it isn't damped.

This is expected behaviour.

If I now press another key (while the pedal is still held down), release the sustenuto pedal (sound of old key stops), press the sustenuto pedal, and release the key:
The sound of the new key isn't damped and continues as well, and this is as well expected.

However, the first key's damper is still up: When I now press and release the old key, the sound still continues.

I was also able to reproduce this result with a longer chain of different keys. Wether this occurs seems to also depend on how long it takes me to press and release the pedal, and the keys. I can however assure you that the pedal wasn't partly pressed when I released it. I also didn't touch any other pedal at all. I'm using pianoteq 7.5.2 on Windows (64 bit). Is this a bug?