The AE2 facade is seen as a full block for vanilla cobblestone walls but not for Framed Wall:

forge-1.16.5-36.2.34.jar
FramedBlocks-2.15.1.jar
appliedenergistics2-8.4.7.jar

I tested this and it turns out this is a bug in AE2.
On the left side I placed the walls first and then the facades on the cable, on the right side I placed the facades first and then the walls (cobblestone wall is vanilla).

I'll report this to AE2 because it's also an issue in 1.18, but there is most likely very little chance of this getting fixed in AE2 for 1.16.

Wow that's such an edge case... but confirmed, after breaking and replacing the framed walls they connect. Thanks!! 👍