An app disappears from your Apple Watch for no apparent reason. Even restarting it doesn’t fix it. Here’s why.
This is why an app disappears on your Apple Watch
If you often use the Music app on your Apple Watch offline, you may have experienced the app suddenly disappearing. You want to launch the player after launching the workout app, but you can’t. What’s more, the Music app that you had as a complication on your Apple Watch face has suddenly disappeared. The player also no longer appears in the app list on your Apple Watch.
This problem is surprisingly old. There are even posts on Reddit and Apple’s support forum similar reports been in watchOS 6. The issue isn’t limited to the Music app, either. Other apps like Weather, Mail, and Maps also seem to disappear from time to time, leaving them unusable on the watch.

This is the cause and solution
You might think that it has to do with a bug in the app management. That other apps are being terminated because the training app is active. However, that does not seem to be the case.
No, it has to do with the fact that you may have the app in question on your Apple Watch, but haven’t used it on your iPhone in a while. Your iPhone automatically deletes apps that you haven’t used in a long time to save storage space. It turns out that those apps are also deleted from your Apple Watch. And that can happen to other apps too.
If the app disappearance is caused by deleting that app on your iPhone, you only need to reinstall the app on your iPhone. After the first start, the app will then appear again in the list of the Apple Watch. The synchronization with the music files on the Watch will then be disrupted. You will therefore have to delete them and transfer them completely again.
You can also do this for app problems
If you’re still having app issues on your Apple Watch, there are a few things you can do to fix them. Here’s a list of them for you:
Hard reset
To force restart, press the side button and the digital crown at the same time. The Apple Watch will restart after about ten seconds. In many cases, this also seems to solve the mentioned problem.
Refresh in background
Another solution is to enable the Background Screen Refresh feature. You can enable this option in the Apple Watch settings. However, this is not an ideal solution because it affects the battery life of your Apple Watch.

Reinstall
Another option is to completely erase and reinstall the Apple Watch. However, that is quite drastic and does not guarantee that the problem will not recur.
Conclusion: app disappears from Apple Watch
The problem of disappearing apps is annoying and occurs because watchOS relies heavily on the iPhone. This can lead to problems. Keep the above solutions in mind and at least make sure that you use an app that you use on your Apple Watch on your iPhone with some regularity.