Transpiling failing in web.browser.legacy Meteor packages

(Duplicated from #11033)

I’m still trying to figure out exactly what’s going on and this issue is mainly to raise awareness/requesting suggestions regarding what else I should attempt to fix the issue.

Recently our staging build has stopped working in IE and after barely any debugging I noticed almost all of Meteor packages were missing transpiling. For example, meteor modules package looks as follows:

And yes, this is the web.browser.legacy version. The other 85 errors you can see in the console are mostly in other Meteor packages.

I can only verify this issue with with 1.10.1 and 1.10.2. The issue did not happen with 1.8.x.

Tested an earlier docker build that I have proof that was working back then (2/3 weeks ago) and this time it no longer works. So we have the exact same source code, with packages installed managed by package-lock, yet we have two completely different outcomes.

I’m honestly out of ideas right now as nothing has really changed on our end and even if this is related with a change in packages (such as babel) I don’t think it’s wrong on my end to assume that package-lock did it’s part.

Any hints on what may be happening?