in https://github.com/Azanor/thaumcraft-api/blob/master/aspects/Aspect.java

Why is it missing? Iter isn't added by Thaumcraft anymore...

hmm. strange, the icon in JEI still shows thaumcraft as mod. So i guess every addon can register aspects but ModTweaker can not access those because the api only returns the inbuilt aspects?

I think Thaumic Bases Unofficial is adding Iter back in. ;)
Also Thaumic JEI is adding those icons for every aspect in JEI, so also not a TC problem here, I guess...

I have thaumic bases, but in this case the api is lacking. Thaumcraft itself is able to process all those aspects, so I should be able to retrieve all those aspects too. my 2 cents :-)

I would need to get the sourcecode of all 3 projects (thaumcraft, ModTweaker and Thaumic Bases) down and fire up my ide to figure out who the culprit is.

Do you have any idea?

Thaumic Bases doesn't use the AspectRegistryEvent and registers the aspects in Post Init (which is too late for crafttweaker to pick them up)...
I already told this the author some time ago...

well than we have the culprit or? :-)