I don't know if this kind of question belongs in this thread or if it belongs in the bug reporting section, but I think I've found what may possibly be a bug:
Basically, it is this: Recipes that show what fuels EnderIO's Stirling Generator will accept as a fuel do not show up in JEI. At least, that's what I think is happening. I recall that this works fine in Direwolf20's 1.10 modpack but also does not show the recipes for fuels it can use, so I'm going based off of that. I did not notice any specific changes that stated certain fuels were disabled for the default FTB Beyond mode for EnderIO generators.
This is a really good modpack as is, I'm off to do further testing as to whether coal will really work in a EnderIO Stirling Generator.
This is in pack version 1.2.1 I don't know if any of the pack devs have scheduled changes for newer versions that I don't know about yet, but I suspect they'll get around to it eventually.
Cheers ...
BrickVoid