Maybe this was intentional, but I noticed that the IXS fusion reactor no longer allows fusing of De-Li6, as it was able to do in prior incarnations of KSP-IE.

I also noticed it doesn't allow He3-He3 fusing, amongst a few of the other fusing modes that things like the Stellarator and such allow (but maybe that's also intentional?).

Apologies if my ignorance of fusion reactions proceeds me. :p

Also modifies the radiator maximum dissipation setting in some odd way, causing one to have to use way more radiators than the heat it actually produces.

I'll look into both issues.

All modes are present. However there's indeed a problem with extremely high WasteHeat production. Please look at:
https://imgur.com/KVwMzX4
He3-He3 mode, so no thermal power - charged power only. 13% active on 90% efficiency should give 65MW wasteheat. For some reason it produces 3.5GW of heat.
@sswelm do you have any ideas? This can be an issue for more than this reactor.

I suspect it is caused by the maintenance wasteheat combined with a mixup of fixed time with power per second

Yep, confirmed and fixed. Closing