You're certainly not alone. I think this started with Simple Delivery and Evri being one of the couriers they use. Evri treat the CIs like Europe, whereas Royal Mail treat them as UK.
Sellers can either opt out of using Evri in their postage preferences:
https://www.ebay.co.uk/ship/prf
Or can try putting your ID on their exemption list to remove blocks:
https://www.ebay.co.uk/bmgt/ExemptBuyers
Not ideal. eBay really need to sort this out.