Davinci Resolve 17 Omitting Clips from within Fusion Clips in Render
The best way to control the order-of-operations is to do all of your compositing in a single Fusion clip; all of your keying and corner-pinning happens in a single Fusion clip, ideally. To set this up, lasso both of the clips on V1 and V2, right-click, and choose &quot;create Fusion clip&quot; from the contextual menu (do this to new, ordinary video clips, not ones you’ve already made into Fusion clips).
But in case you are trying to preserve some amount of edit flexibility, or have another reason to keep the Fusion clips distinct on the Edit timeline, I suspect that the issue is an order-of-operations problem related to the Color page.
I think that when Resolve sees two Fusion clips stacked on top of each other, it processes the
bottommost clip’s Fusion effects first, passes the V1 Fusion to the Color page, then calculates V2 Fusion and V2 Color.  I could be wrong about bottom-first processing, but it definitely does send to the Color page before returning to Fusion to handle the second clip, whichever one that actually is.
So, it’s likely the Color page that’s messing up the alpha channel. You could try opening the topmost clip on the Color page, right-clicking anywhere in the node tree background, and choosing &quot;add alpha output” from the dropdown menu. In previous versions of Resolve, it was necessary to inform the Color page that it should output an alpha. Since your alpha is coming from Fusion, you'll also probably need to right-click in the background select &quot;add source,&quot; and wire the new green triangle on the left to the blue alpha output on the right.  This is the manual way to tell the Color page to &quot;pass through&quot; an alpha channel.
Resolve’s image pipeline undergoes frequent modification, especially with respect to alpha channels, and the way different Edit page containers prioritize and calculate (including premultiplication).  For the most part, it has improved greatly over the years, but you'll still run into situations where it doesn't do what you'd expect. The developers are great about incorporating user feedback, but on the other hand, that's why the image pipeline is in a constant state of flux.  And since the codebase changes more frequently than the documentation, sometimes the order of operations isn't clearly defined…
Which is why it's still best to contain all of your Fusion compositing in a single clip.
One last thing that might help, and I always forget to recommend, because it's a new feature; Render In Place.  You can render either (or both) clips to disk, which will both speed up playback and help you troubleshoot, and depending on your requirements may be a sufficient solution itself.  Right-click on either clip in the timeline and select the option from the dropdown, but be sure to select a codec which supports alpha channels, such as ProRes 4444.