If you still have this issue, here's the fix:

1. Go to your Heroku app, copy the DNS-target for your apex (root) domain (the one that looks like "stark-horse-mrp4jeowu9yvwpnnma32x6hd.herokudns.com")

2. Go to https://www.nslookup.io/, put the DNS target

3. Go through all 5 tabs at the top (Cloudflare, Google, Authoritative) - wherever i you see "A" records - I could only see them in "Authoritative"

4. Copy first A record

5. Go to CloudFlare, replace the flattened CNAME / A record you have for your apex domain with this A record IP you copied

6. Your website should work now, but do the same with the other A records you found as well

7. You should probably revert back to whatever you had in Cloudflare, as soon as Heroku is back up