The first sentence sounds like CLN is at fauilt in this case. To be clear, the change in the specification was initiated 3 years ago by the LDK team, and LND have simply not followed the spec change.

CLN has a backwards compatibility shim in the invoice generation intended to be there until the rest of the network upgraded to also default to 18 blocks. After 3 years we went and removed this backwards compatibility shim, expecting all nodes to be long updated, but LND had not, and so we re-added that shim.

So, while CLN users would encounter this problem, despite behaving correctly, the larger share of LND nodes out there, forced our hand to take a step backwards rather than forwards...