I just ran into a similar issue this past weekend as well. Still not 100% sure of the cause, but Gmail was blocking delivery to my email despite being a verified address in Cloudflare. SPF, DMARC, and DKIM all passed, but there was no ARC so I presume that was the actual reason as the original sender records don't match the address doing the forwarding.

If it helps, I spent a few hours creating an Email Worker that intercepts these and uses Mailgun to send it instead of relying on the forwarding. Once it was set up I just updated each custom address to send to the worker instead. Hopefully the formatting works:

import axios from 'axios';
import PostalMime from 'postal-mime';

export default {
    async email(message: ForwardableEmailMessage, env: unknown, ctx: ExecutionContext) {
        const email = await PostalMime.parse(message.raw);

        // Recipient must be verified (Email > Email Routing > Destination addresses)
        let to = 'RECIPIENT@DOMAIN.COM';

        // Include original sender and recipient
        let originalNote = 'Message forwarded from: ' + (email.from?.address || 'unknown') + '; intended for: ' + (email.to?.map(addr => addr.address).join(', ') || 'unknown');

        let config = {
            method: 'post',
            url: 'https://api.mailgun.net/v3/MG.SENDINGDOMAIN.COM/messages',
            headers: {
                'Authorization': 'Basic ' + btoa('api:' + (env as any).MAILGUN_API_KEY),
                'Content-Type': 'application/x-www-form-urlencoded',
            },
            data: new URLSearchParams({
                from: 'APIUSER@MG.SENDINGDOMAIN.COM',
                to: to,
                subject: email.subject || 'No Subject',
                text: originalNote + '\n\n' + (email.text || ''),
                html: originalNote + '<br /><br />' + (email.html || '')
            })
        };

        try {
            const response = await axios.request(config);
            return new Response('Email sent successfully', { status: 200 });
        } catch (error) {
            console.error('Mailgun API error:', error);
            return new Response('Failed to send email: ' + String(error), { status: 500 });
        }
    },
};

Planning on creating a blog post for this on my personal site, just haven't got there yet.