Changing CNAME from DNS-only to Proxied results in HTTP/1.1 409 Conflict

On one of our domains we have cname records that point to herokudns.com DNS targets that were generated with Herokus ACM.
All the cnames are currently with Proxy Status: dns-only. This works ok, but I get complaints that http requests are not automatically redirected to https.
So I set the ‘Always Use HTTPS’ to true in Edge certificates section.
If I now set one such cname record to proxied, it no longers directs to the heroku app.
I get this with curl:

curl -v -L http://my-domain.com

* Trying xxx.xx.xx.xxx:80...
* Connected to my-domain.com (xxx.xx.xx.xxx) port 80 (#0)
> GET / HTTP/1.1
> Host: my-domain.com
> User-Agent: curl/7.79.1
> Accept: */*
>

* Mark bundle as not supporting multiuse
< HTTP/1.1 409 Conflict
< Date: Fri, 16 Jun 2023 14:40:08 GMT
< Content-Type: text/plain; charset=UTF-8
< Content-Length: 16
< Connection: close
< X-Frame-Options: SAMEORIGIN
< Referrer-Policy: same-origin
< Cache-Control: private, max-age=0, no-store, no-cache, must-revalidate, post-check=0, pre-check=0
< Expires: Thu, 01 Jan 1970 00:00:01 GMT
< Server: cloudflare
< CF-RAY: 7d83ce627fe00e84-AMS
<
* Closing connection 0
error code: 1001

I don’t even know where I could start to look for solution, any hints?

Believe this was solved via a second thread @tech39?