'listenTo' is undefined

I’m trying to upgrade our okta-signing-widget from 3.1.5 to 3.4.1 via Yarn. 3.1.5 works great, but if I move to a version higher than that (tested 3.3.3, 3.4.1 and 3.5.0), our login fails to render and console reports TypeError: Cannot read property 'listenTo' of undefined. In version 3.4.1, This error is reported in okta-sign-in.entry.js at line 34529.

If I open the file, I actually find the line at 32282: this.listenTo(Router.prototype, 'all', this.trigger); located in the bootstrapRouter function within the function var OktaSignIn. The error appears to be related to the strict mode definition of this, because if I remove the use strict declaration from above OktaSignin, it seems to work as expected.

Any ideas on what we need to do differently to make this upgrade work? Thanks!