This is just nodes decoding the invoice assuming a 9 block cltv delta if not explicitly specified with a c field in the invoice, but the BOLT 11 spec was changed to 18 blocks over three years ago, so that assumption is bad. If your node is decoding incorrectly, it's time to update it to the current standard!

This has been patched with CLN v23.11.2 by always forcing inclusion of the c field so there can be no ambiguity. It would be nice to relax this at some point so that default invoices and QR codes could be more compact. If you're facing this issue with nodes or services unable to pay a CLN 23.11 generated invoice, please update to v23.11.2.