Strange terminology with "bulk voltage". In Bulk, battery voltage rises until it gets near the Absorption voltage setting. (typical 14.6v for a 12 and 29.2v for a 24.) Then amps start to taper while the charger holds the voltage at the Absorption voltage.
Anyway, this is for a 24v system? So 25v would be like 12.5v for a 12v system. That would not charge a 12v battery, so 25v will not charge a 24v battery.
The charger thinks it has a 24v battery there (as set?), so keeps on going to something more like 29v. 27 is the lowest it will do in 24v mode as stated in the OP, which makes sense.
You need an adjustable voltage charger that you can set the voltage and it stays at that no matter what the battery system is. PowerMax has such a thing. Some others might too but don't know.
EDIT--Oops their 12v only goes up to 16.5v and their 24v starts at 26v.
https://powermaxconverters.com/product/pm3-12v-series/Maybe you can use the 24v job and add a resistor so it does under the 26v. I am not a tech so can't say. If you can do that, maybe you can tame the Xantrex the same way to stop it going above 25.
