First Class on those VT services costs more than Standard.
On West Midlands Trains, which is where the phenomenon in your screenshot is observed, this is very common. I think the main cause is that the cheap Standard Class Advances represent excellent value for money, with savings of as much as 90% over some of the competing operators. This means that these tickets fly out of availability quickly when they go on sale. First Class on West Midlands Trains has very low differentiation with Standard Class. On a susbset of their rolling stock, the 350/3, one could argue that Standard Class is actually more comfortable. Thus, demand for their First Class advances will be considerably lower, and initial sales much slower.
In circumstances where First Class is the cheaper option though, I can't think of any reason not to buy it. Even if you don't want to sit in First Class you can simply sit in Standard with that ticket.