First thanks, you are right, tun/tap is built-in. I wouldn't have expected this.
Second, could you please point me to .config? I must be blind, I cannot find it.
I see the strange behaviour as you. However it doesnt't bother me much, because I write a start script anyway.
I can only imagine there is something wrong with the binary of OpenVPN Installer. I found a hint to this:
However I tried the workaround with the symlink to /system/xbin/bb, it didn't work neither.