The note for http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-rdgGrp.html contains the following text:
Note that only one lem element may appear within a single apparatus entry, whether it appears outside a rdgGrp element or within it.
But there are a number of examples of
app containing multiple
lems in http://www.tei-c.org/release/doc/tei-p5-doc/en/html/TC.html#TCAPSU
I have no idea what it would mean to have multiple lemmas, so the note seems right to me, but unless I'm mistaken, either the examples have to be fixed, or the note has to go.