I find the same as you, and get a similar problem whether I'm using a voucher or not. And the issue is the same on UVF-GND as another example, which is not short of availability.
I suspect it is a problem displaying fifth-freedom flights, maybe down to their pricing, rather than being related to voucher limitations, so I imagine you can get round it by calling them up.
[I didn't know this but the avios.com and BAEC call centres are one and the same, at least at times, so I doubt they'll struggle to see that the availability exists. I got through to avios.com 2 minutes before closing and felt guilty I wanted to make 3 bookings, but the agent was on a continuing BAEC shift, not an overrunning avios.com one.]