Because your web appbuilder developer edition is v2.12 too old, not match current portal version.
Cause security issue, authentication failure. It always try to login then flashing.
https://support.esri.com/en-us/bug/web-app-created-using-arcgis-web-appbuilder-developer-e-bug-000146860
Where to find your developer edition version number?
flashing is because of this web adaptor subdomain gis1.xxx cause security issue,
Solution is use machine name instead of sub domain name web adaptor name in the portal item URL
Every open it, user need to login portal, there is no way to skip this login process.
