I have the following issue: Sometimes work tasks appear before I come to work or after I leave (before 9 am or after 6 pm) and someone else (working more hours

I have the following issue: Sometimes work tasks appear before I come to work or after I leave (before 9 am or after 6 pm) and someone else (working more hours than me) does the tasks and my manager gets complaints about me that I'm not around. I can certainly handle the workload, and the tasks usually can wait, but there are people staying 12 hours at workplace ready to do them or complain that I'm not present. So far my manager was on my side because I wasn't informed about these tasks beforehand, but this still causes a lot of stress for me.