Hey, Often there are manufacturer's like Xiaomi who allow you to lock an app in the recent page thus even if you clear the apps in the recent page that particular app would stay there. So check if there is a similar option for your device. Also go into Settings>> Installed Apps>> (App you are looking for)>> Check the autostart option if there is any. Also in the same page Check the option of battery optimization to no optimization for the particular app. Xiaomi have a really aggressive battery saving optimization feature which force closes apps running in the background, granting autostart access to the same app should solve this problem.

Best Regards, Mamoon