A few things to check and comment on here.
First, do you see the same thing with LDA or GGA, i.e. is it specifically a problem of MGGA? If it is, it makes it a bit more interesting to investigate.
However, apart from perhaps a somewhat instructive tutorial example, the calculation doesn't really have any value. The transmission spectrum of any bulk material should really just be counting the states at each energy; in 1D this is trivial and gives you an integer staircase, while in 2D and 3D it's a bit more complex due to k-point sampling, but conceptually still the same. To make an interesting calculation, one should create an extended device with some scattering centers. If also in this case the transmission is non-zero in the band gap, I agree there is a big problem, but we have never seen that.
Now, I am not trying to minimize the potential issue, we should probably look into it, but it's also not a serious problem if this is isolated to the bulk transmission, as that is generally not something you need to compute; we support it mainly for legacy reasons. Or, to some extent for educational purposes, so again, it should of course calculate it correctly. Thus, if you can check it with LDA also, it will help us a bit.
