CX system rule is probably “if current flight is more than 5 hours then print BP for next flight” and can’t handle checking two conditions at once.
To the points above, regardless of the OW rule (which is indeed LH to SH, and LH defined as 5+ hours), I think something along the lines of what
sxc says is closer to truth. I don't think it's a one off mistake at least, since we frequently get people on here doing CX-CX connections where they technically shouldn't get the lounge pass, but they do.
Where CX / KA actually DO seem to enforce it is if one of the flights is OW partner metal, instead of CX/KA. Here we have people saying they were denied the lounge, which is in line with OW rules.
So I think the bottom line is if you're connecting CX->CX, you definitely get it if one of your segments is LH. Even if the other one is LH too and technically outside the OW entitlement.