The struct for RWStructuredBuffer has a size limitation?

I made a struct for RWStructuredBuffer, as below
struct particle
float3 src_pos;
float3 dst_pos;
float3 pos;
float3 vel;
RWStructuredBuffer Output : BACKBUFFER;

but the last variable “vel” can’t be added to itself while the “pos” works properly.
and the weird thing is if the “vel” add “1” to itself every frame, it works properly too.

and if I change all the float3 to float2, everything works fine.

so, does the struct have a size limitation?

the attachment is the patch I made

CSNewtonTest.rar (7.8 kB)