Modal inside modal: how do you resolve the issue of the internal modal's backdrop extending to the full vertical outline of the external modal? In the past I've resolved a similar issue for a spinner inside a quick-action modal by applying slds-is-relative on the internal component, but this doesn't seem to work in this case. Thanks!
Modals are not designed to be placed inside other modals. Without seeing some sample code, it's hard to say how you'd fix this, but markup for one modal definitely should not appear within another. If you're using Aura, use the lightning:overlayLibrary to properly layer modals. There's no such equivalent yet in LWC, but you could build your own without too much effort.