This is not a bug. The window is created fine the first time you type the command but you can't see it because you used the -h switch to hide it, so mirc is doing what you told it to. If you type the command a second time, mirc places the window at position (1,1) which is also normal, because -C is only effective when you create the window (ie in the first command). The reason it doesn't remain hidden the second time is that you've also specified the -a switch, which apparently takes priority over -h when the window already exists.
I assume you used -h not to hide the window but to hide its switchbar button. If so, you can now use the -w switch with 0 as the parameter to hide the switchbar button (as well as the treebar item):
/window -aBCdfow0 +deL @splashscreen 1 1 512 384
