decimal point is truncated when using draw render target2D

Hi, guys. Have you met a problem like this when using Draw material to Render Target?
I try to save some data in renderTarget2D and I make a material and use it to ‘ping pong’ update the two renderTarget2D in blueprint (see the picture)
Actually, I increase every color component by 1 every frame using a custom node like this:

float3 Position = Tex.Sample(TexSampler, UV).rgb;
return Position + float3(1,1,1);

but the things: the number I save in renderTarget2D will stop at 2048 and could not increase anymore. I am sure I use RTF RGBA 16f format for my renderTarget2D.
I search it on google and only see one guy has a similar problem: (this is in Japanese)
What case the problem, what’s the thing behind?
Thanks in advance.