Implementing the workaround right now I've noticed that another issue with the separate invocation of pacman -U is that any dependencies installed via "makepkg -rs" are removed and then re-installed during installation with pacman, while omitting "-r" will leave makedeps.
pamcan -U deals with dependency resolution.
Yeah, but lets say you want to install foo with depends=(bar baz lulu) makedepends=(this that whatever) makepkg -rs installs bar, baz, lulu, this, that and whatever, builds the package, then removes all of them. Subsequent pacman -U then re-installs bar, baz, and lulu. Even if the packages are small with no side-effects via install scripts, that is inefficient. If they are large and/or have side-effects (e.g. updating system databases, font caches, etc) then it is very inefficient.