Original issue: BluSunrize/ImmersiveEngineering#3181
This seems to be caused by some code in FMP that assumes vanilla's canConnectRedstone can't handle the bottom side properly. The current canConnectRedstone is added by a Forge patch, MCPBot indicates that a vanilla method of that (MCP) name existed in 1.5 (mh canConnectRedstone), which would fit with the code having been added 5 years ago.
@covers1624 asked me to open an issue here, otherwise I'd probably have opened this in the FMP repo.

I thought I fixed this a long time ago, but maybe I never opened a PR, or the change was lost at some point.

Will be fixed by: TheCBProject/CBMultipart#149

This is an FMP issue. I will look into it and push a fix.