@Rabia BEZZAZ, Thanks for posting in Q&A. From your description, it sounds like you're experiencing some challenges with finding applications in the private store for Android phones. Based on the information I found; it seems that this behavior might be expected due to the way the system is designed in Managed Google Play.
https://support.google.com/googleplay/work/answer/9495634?hl=en
Note: None-Microsoft link, just for the reference.
For private store apps, it can indeed be more complicated to find them. Sometimes, just typing the name of the app might not be enough, and you may need to type the package name. This is because the search functionality in the private store might not be as robust as in the public store. You can feedback to Google Play Store to see if it can be changed in the future.
Hope the above information can help.
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