We have the same issue and we could narrow it down to --minifyjs as well.
In our case, the storage plugin does not work properly and events like (ionBlur) or (keyUp) don’t fire anymore.
<ion-input type="text" (ionBlur)="checkValid()" [(ngModel)]="address.name" placeholder="Location Name"></ion-input>
Without --prod or --minifyjs, or in the Browser, everything works like a charm.
I found this:
“It also ignores plugins/imports which you have included but haven’t used.” in this post and that got me curious.
Maybe it ignores some of the dependencies, as you mentioned as well, but we are still not able to solve this problem 
If you found something out, please let me know!
Edit: With ionic 3.6.0 and the same code, everything worked as well. With 3.9.2 it doesn’t.