The issue is that you have renamed the VSP pump speeds that shouldn't have their names changed. Most of the VSP speeds are hard code associated with certain functions.
When I look at that VSP Adjust/set speed page on my system I see:
You have renamed the Pool Heat speed Full Speed and assigned it 3450 rpm, the full speed of the pump. When any of the heat devices, Pool Heat, Spa Heat, or Solar Heat are enabled and actively heating the system will not let you set the pump speed less than the associated Heating speed. In your case you have the Pool Heat speed setting set to 3450, so if you select any speed slower than that it won't let you do that.
The system hard codes Pool heat to VSP speed setting 5 regardless of what you name speed 5 to.
Go back into your VSP speed setup table and name the speeds back to the defaults, and then set your Pool and Spa heat speeds accordingly. Here are the speed labels from my system:

In general the first six speeds are all assigned to functions and you cannot rename them. If you have the DIP switches set up to not have a cleaner, or spillover functions then speeds 3 and 4 can be renamed and are available for general use. Speeds 7 and 8 are the only ones that are generally free for relabeling and reassignment as they are not associated with anything in most systems. In my case I have a solar heating system and speed 7 gets automatically associated with the Solar Heat function, but that's not true for most people.
Change your names back to the proper default names above, set your pool heat and spa heat speeds to what you want, and move your Full speed (3450 rpm) speed to speed 8 and you will be okay.
You will never be able to command a speed less than the Pool heat speed when the heater is firing, but you can set the pool heating speed to what you want if you label your speeds correctly. E