Snap apps keep asking installing ubuntu-app-platform and connecting platform interface (stale mount namespace)

The problem is that my app keeps giving me the following error message:

$ ubuntu-webapp
You need to connect this snap to the ubuntu app platform snap.
You can do this with those commands:
snap install ubuntu-app-platform
snap connect ubuntu-webapp:platform ubuntu-app-platform:platform

Using "snap interfaces" command, it shows that the platform interface is auto-connected after the app is installed and it does not need to have a manual connection. The ubuntu-app-platform package is also installed on the system

I have a sample project using the platform interface for launching a website. My sample app is at:

https://github.com/ubuntu/snap-tutorials-code/tree/master/webapp-ubuntu/final

To produce the problem:

1) change the confinement from strict to devmode
2) build the application and run it
3) change the confinement from devmode to strict
4) build the application and run it

You will be able to see the problem. The only solution is to use the following command:

sudo /usr/lib/snapd/snap-discard-ns ubuntu-webapp

to clear the namespace firstly, then followed by a re-installation. It starts to work correctly. If there is no re-installation, even the namespace has been cleared, the application still crashes

If I do not do a re-installation after clearing the namespace, I get a Segmentation fault like:

=================================================
liuxg@liuxg:~/webapp-ubuntu$ ubuntu-webapp
QGtkStyle could not resolve GTK. Make sure you have installed the proper libraries.
qml: Loading webview on /home/liuxg/snap/ubuntu-webapp/x3/.local/share/snap
QQmlExpression: Expression file:///snap/ubuntu-webapp/x3/usr/share/webbrowser-app/webcontainer/WebApp.qml:288:25 depends on non-NOTIFYable properties:
unity::action::qml::ActionManager::globalContext
Segmentation fault (core dumped)
=================================================

Thanks & best regards,
XiaoGuo