Thank you for reporting this. At first glance this looks like a bug/regression in 2026.3.0. I'll have to check in more detail myself before I can say for sure. But your code is also incorrect, the NBIT_TLX_SELECT2 bits are internal and not for what you think they are. You must use NBIT_TLX_SELECT instead. But they are currently malfunctioning for me on Windows in 2026.3.

Until I am sure, I will not yet classify this as a bug.

Cheers,
Ferdinand

In 2026.2.0, this code randomly selects and deselects tracks in an all timelines for the active object. I.e., you can spam-click the script and see the track selections 'jump around'. In 2026.3 it does exactly nothing (at least on my Windows machine).

import c4d
import random

def main() -> None:
    for track in op.GetCTracks():
        bit: int = random.choice([c4d.NBITCONTROL_SET, c4d.NBITCONTROL_CLEAR])
        for tl in (c4d.NBIT_TL1_SELECT, c4d.NBIT_TL2_SELECT, c4d.NBIT_TL3_SELECT, c4d.NBIT_TL4_SELECT):
            track.ChangeNBit(tl, bit)
    
    c4d.EventAdd()


if __name__ == '__main__':
    main()