This issue has been fixed for a while now. Set the task to repeat every month on the last day.

Test ^jan31 *every month on the last day

Well, I have a task that I should do every 29-th, and it is broken

Hi aleksi,
Thanks for reporting that and sharing your example, and I'll pass that on to the development team. We appreciate you reaching out. 💙

Why don't you set four different task:
the first the same you have ^29 jan *every year
the second, third and forth, the day you want on february on day you want on non leap year repeating every 4 years

Please, bear with my poor english!!

(correction)
the first the same you have ^29 jan *every month