Maybe it’s related to the Ionic 3 Codebase? Did you migrated everything like it’s documented in the Migration Docs?
- https://ionicframework.com/docs/building/migration
- https://github.com/ionic-team/ionic/blob/master/angular/BREAKING.md#tabs
Ionic 4 has some breaking changes in case of usage of Angular Components, etc…
Cheers
Unkn0wn0x