I will try to calculate a little; please correct me if my assumptions are wrong.
So, you are working with V3-V4 primers (341F and 806R), and currently trimmed your sequences at positions 17 and 20. So, your expectation of 430 is based on the math: 806-341-17-20 = 428?
Here is the link to the comment of the dada2 developer. According to it, you should expect at least 2 groups of sequences, with one group shorter by approximately 20 nt than another group.
In addition, any reads that fail to merge will be discarded by dada2. You can also go for additional filtering based on taxonomy classifications if needed.
I was confused as well with my first dataset, which was also sequenced with V3-V4. Here is a nice graphical representation of V3-V4 region length.