I'm running Kstars 3.5.2 on a RPI4 and recently my asi290 has been losing the guide star and has trouble reacquiring. I will be happily guiding and all of a sudden it will lose the guide star and is unable to reacquire it. It also sometimes fails to acquire the guide star when it's doing a guide calibration. I've set gain to 100.
I suggest 1st identify if this is a hardware or software problem. Please close KStars and try a guiding session using PHD2 and try to connect using direct drivers (not through INDI). If the problem occurred again, it is a hardware issue or maybe a corrupted image that can be fixed by reimaging on a new SC card.