Bloodmoney spell cannot be cast in quick succession

I noticed that now a few times, no major issue, but I just put it here anyway. So when I have a lot of prisoners and use the bloodmoney spell in quick succession on captured enemy units in my prison, it sometimes happens (unfortunately, not always, so I am not sure what triggers it) that I cannot use the bloodmoney spell on a captured unit as long as the spell is still working (casting, transforming time) on a unit of the same type. As soon as the bloodmoney still finishes the other unit, I am able to click on the next unit (i.e. a red border shows that now the spell can be applied on the unit) to cast the spell on it.

E.g., if I have a lot of lvl 1 templars captive, and I often see that I cannot transform them in paralel but have to wait until one is completely transformed.