Whenever I die as Karthus I'm pretty sure I'm supposed to be able to cast spells for 8 seconds afterward. However, if I cast Requiem (3 second channel) upon death, it no longer lets me cast spells. I should have another 5 seconds to cast. What's up with that?

Can I get some feedback on this?

It's almost certainly intentional - probably just a little quirk that was too wordy for the tooltip. In most circumstances you probably don't want to cast it first anyway though.

I doubt this is intentional, seeing as how you are free to cast spells for under 5 seconds and then cast requiem for better effect.
You said it yourself, why cast Requiem first? It definitely isn't an option with this bug.

this has been around for ages, casting requiem will stop you from being able to continue casting while dead. its really not THAT big of an issue, because you should be casting requiem at smart times, and usually it will already be on cd before u die anyways, alternatively it wont kill anything so there is no point wasting the cd.

it's intentional
once you cast it you're no longer able to cast spells afterwards, also, you cant start it after 5 seconds either (for more obvious reasons)

I dont really remember the reasoning for it, but I do remember hearing it was intentional all over the place