Do you have an example request that caused?
In a couple of spot tests I couldn’t get a 400.
I looked up three users (via login=foo&login=bar&login=baz, one of the logins being a banned Twitch User)
I got the expected two entries in the data array, the third being missing as banned
Edit: your error would suggest that, in the case of id=x&id=y one of the ID’s is a word instead. Or one of the login’s is invalid/doesn’t meet the formatting.
Generally speaking you might be able to get away with 1-to-1 look ups. Since generally it’s tough ot eat the rate limit, if running concurrently.