It looks as if email forwarding isn’t to be trusted any more. you’re depending upon it, then before you expect to get any message of importance, test it. Do not be surprised if it suddenly stops working for you. Organizations are continuing tightening email security.
A new article describes how to tighten email security for any custom domains you may have hosted at another email provider’s site (e.g., Google for Work, Apple iCloud+, etc.): https://talk.tidbits.com/t/need-to-check-dmarc-settings-for-custom-domains
As to why: due to recent increases in fraudulently spoofed email, the big email services (gmail, yahoo, outlook, and apple) as well as a lot of other organizations are tightening incoming email security. As a result, other organizations which provide email to large numbers of people (universities, large businesses) are having to update their outgoing email security
Using strict SPF, DKIM, and DMARC (esp. DKIM) makes message forwarding very difficult. I suspect most sites which currently offer mail forwarding will simply stop doing so. [Possibly without announcing that they’re doing so, which is why you need to test any that you want to continue to depend upon.]