Really. Got any reliable sources on that, using a reasonable definition of "voluntary", based on sources other than veteran war stories[0] and film plots? :-)

(a reasonable interpretation of "not voluntary" includes all people who were forced to join the army because it was the only viable career-option for their socio-economic class)

I'm not saying it doesn't happen, but it seems highly likely that the perceived frequency of these occurrences must be extremely inflated because it's been in the interest of war propaganda anywhere since forever, to pretend that knowingly volunteering for a suicide mission is not only perfectly sensible, not only extremely noble, but that this is in fact what you already signed up for, so why are you still standing here?

Then there's the question whether this is even relevant in the context of evolution. Soldiers have to be trained and drilled intensively for the explicit purpose of artificially altering their learned instincts which seems to me quite solid evidence of the contrary; that this behaviour is exactly not part of human nature, and that humans by default will not act like this unless you train them to.

You can probably train crows to suicide-attack humans as well.

[0] I realise leaving out this group leaves the data a bit thin on the ground, but given the extreme biases involved, it's useless data. So many stories people would rather not talk about, to name just one bias. Or stories altered after the fact (like they actually drew lots but tell it as voluntary because they are thankful and know it could've been them just as easily).