I see why this has happened. The taxonomy string is not stored in a fixed-width format inside the artifact and was getting an arbitrary column title (ta1->ta7). A quick work around that should work now is plot_bar(phyloseq, fill="ta2").
I just pushed an update that should more carefully import the taxonomy and explicitly name the columns such that plot_bar(phyloseq, fill="Phylum") should work. Just reinstall using devtools::install_github("jbisanz/MicrobeR") to get the newest version.