@Jurgen Cassar Thanks for your patience and sharing the requested details.
I am summarizing our offline discussion and posting the same as an answer.
Issue: unable to see a particular web app from the app service blade and they are able to see, access the webapp from the recent accessed resource list in the portal.
Things tried:
Requested to list the app service under the subscription either from CLI or using the Rest API @Jurgen Cassar confirmed that they are able to see those missing apps from CLI.
Resolution:
We performed a resource sync operation @Jurgen Cassar confirmed they were able to view the missing webapps (from portal).
Please accept as Yes if the answer provided is useful, so that you can help others in the community looking for remediation for similar issues.