Second extruder skips prime tower only on first layer

[RavenWorks 0]

I have a Flashforge Creator Pro 2, and printing with Cura is mostly going well, but I'm having one small problem that's making prints tend to go wrong right at the start:  my second print head doesn't visit the prime tower on its first layer.

 

I'm printing with PLA and PVA; I put down a raft of PLA so the PVA supports have something easier to adhere to, which is great in theory, but it means there's several layers of nothing but PLA printing the raft while the PVA just waits for the first time it'll be used; even with a lowered standby temperature, it takes a moment for the PVA flow to turn out well after that.  In theory that's what the prime tower would address, but I just finally noticed: the first time the PVA head comes out, it goes directly into printing the supports, ignoring the prime tower completely.  On all subsequent layers, it comes out, adds to the prime tower, and then adds to the supports; but the first layer is when it really needs the prime tower, since it's been waiting so long, and it skips it completely, and as a result my first layer of PVA tends to have like an 80% chance of just being spaghetti.  (If it survives that first layer, all the subsequent ones are fine, so the prime tower clearly does its job, when the printer isn't skipping it!)

 

Is this a bug resulting from combining a prime tower with a raft, maybe?  Has anyone else run into it?