We are not sure what happens but it has been impossible to create a video with alpha transparency that Vmix could use, it is simply impossible. no matter what format or codec you use it is really impossible. the transparency is visible in after effects and premiere but Vmix can only see the alpha if this is rendered from adobe or another application. The problem becomes worse when we realize that you cannot render the alpha from the native transitions of resolve, it is simply impossible and at the same time incredible that something that seems so simple is not implemented yet. I hope the development team will check this. What the hell happens with the alpha renders and with those transitions?, we love DR an Blackmagic but we had to go back to using adobe for this!
System specs: Resolve Studio version: 18.5.1 OS: Windows 11 CPU: Intel core I5 RAM: 32GB GPU: GTX 1070
the transparency is visible in after effects and premiere
implies that this is not really a resolve issue
although i agree that exporting video+alpha from resolve is often not exactly a day on the beach. for example ,as you said, with standard transitions its impossible.