If it was from unsupported apps, I would understand. But Microsoft is supposed to still support Windows 10 mobile for a few more months. It's not normal that so much Microsoft apps have this problem. I think the reason why Microsoft would want to do this is to stop supporting this system faster, because there is a cost to support it and maintain the servers up. And because they don't sell any device anymore, it's a cost with not a single revenue from this system anymore (or a very very small revenue). So they would want to kill Windows 10 mobile to cut the costs associated with the support of this system.
Maybe my theory is wrong and the reason is simply incompetence, publishing updates that cause such problems. But I don't think Microsoft developers are such incompetent.
The difference since I blocked background work for the apps is impressive. Before that, I had to recharge my cell 2 or 3 times per day. I had to recharge it before night and at the morning because the battery was very low after doing absolutly nothing during night. At the end of my working day, the battery was near to be empty. Sometimes it was empty. Very annoying. And my cell is not so old, I bought it early 2018. Now, my cell lose only 5-10% battery during night, and I can use it as I want during the whole day without any problem. Much better.
I don't know what kind of problem could cause such battery drain, only from Microsoft apps. I hope they will fix it if it was an unwanted error.