chrisonline Sorry for exaggerated explanations.
So, your suggestion to read that website was good. My battery settings turned out to be correct. But on Redmi there is something like 'recent app tray' and it turned out that there are hidden settings there, I was unaware of. Using these settings one can lock an app to the result that notifications do not disappear if cache has been cleared. It seems to me now that COL Reminder notifications became also stable i.e. they do not disappear suddenly without any visible reason as before, but I will have to test this longer to be sure.
Thank you very much for your help!