I've recently been playing around with being a pacifist, which forces you to use the defensive wars only policy. The policy indicates that you can only make claims on rival empires while in a defensive war with them. However, I've found that this isn't always the case. Sometimes I can make claims and sometimes I cannot, and I can't identify what additional factors may be allowing or preventing me from making claims.

Can anyone explain to me under what circumstances I can claim territory during a defensive war, and in what circumstances I cannot?

if they are evil (warlike) you can make a claim to change their policy

if they have a claim against you then you can claim back... a pacifist race is all well and good until you find the skaven are next door neighbours

I don't think you can make claims in a defensive war against Determined Exterminators, Devouring Swarms, Driven Assimilators, and Fanatic Purifiers. In those cases, you shouldn't need claims anyway. You should be able to make claims in a defensive war against anyone else.

Likely a bug then. I'm pretty sure that the game script can_add_claim located in (I think) Stellaris/ common/ scripted_triggers/ 00_scripted_triggers.txt is meant to control this, and I don't see any other grounds in it for disallowing making claims on an attacker during a defensive war. By the way, have you tried saving and reloading during a defensive war?

Yes, I still have saves for several of the wars during that game. I can make claims just fine in the first war, while in the subsequent wars the claims menu is greyed out with the message that there are no empires that I can make claims on.

are you in a federation with someone?

Just a defensive pact