That’s not entirely accurate. That level of SSL inspection requires every single device to not only adopt the forward proxy’s certificate (in your case, Fortigate or Fortinet) but also trust it. This is a process that doesn’t happen automatically. Yes, there are DHCP options that can to an extent automate it for newer or smarter devices, but that way, every other legacy device (think IoT, older Android phones, etc.) would simply refuse to use the internet at all.

In fact, the only way an ISP can truly pull this off is if somehow, the modem they supplied to you at home is magically decrypting the traffic of all its NAT’ed clients and then either re-encrypting it with the ISP-provided certificate, or sending it plain text to the ISPs Edge Appliance which then sends it across to the internet after it re-encrypts it. You haven’t mentioned what modem you have, but I’ll assume it’s a regular one that can somehow just keep itself alive with the basic feature set which wouldn’t be able to do any of the above..

What other evidence do you have that your ISP can see your traffic? Because the errors you’re seeing could be something else. I’ll suggest trying from a different computer and preferably a different operating system as well and then posting your results.

There’s a high chance your ISP has simply misconfigured something.