Xiaomi's MIUI is famous (or infamous, depending on your perspective) for the widespread changes it does to the Android OS. One of these changes is the aggressive battery management, which prioritizes battery life of the device with a controversial side-effect of killing off apps in the background. This does lead to apps skimping out on delivering notifications to your device when ideally they should have been delivering them.
While MIUI's battery optimizations do shoulder most of the blame for these skipped notifications, they are not the complete cause. If turning off the battery optimizations does not fix the issue of missing and broken notifications, then your issue may be the same as was experienced by Redditor FabioCZ on his Xiaomi Mi Mix 3 on MIUI 10. Notifications for apps such as Discord and Google Voice were apparently instantly removed because of "REASON_ERROR", an error code which basically means that MIUI is unable to display some content within the notification, and hence, cancelled the notification from being displayed.
FabioCZ then turned towards fixing the issue, and developed the Notification Repeater app for MIUI 10 devices. This app listens for incoming notifications that are dismissed with the error mentioned above, and then repeats these missed notifications in a basic manner in order to ensure that they do get displayed on the device. The new notification is just a clone of the title and content of the original notification, but it strips out the styling to achieve its purpose. As a result, features like quick replies are not supported.
The developer has also pushed an update to the app that includes instructions for enabling notifications, ability to trigger test notifications and to allow vibration, light and floating notification functionality to now work. The app is also open source, so you can take a look at the app yourself to ensure that the code does not do anything nefarious.