Thanks for qiiming 
in. Just came across this thread, providing the same explanation. Sorry for the duplicate thread.
In Dada2, ASV ids are actually MD5 hashes of sequences itself. So, the same sequence will always get the same ID.
I actually didn't know this!
But do I even need to merge anything (now that I know that ASV as hash based and I run dada2 with same params) if a just want to predict the metadata of a new sample? As I understand, I would need only to run the already established model against a new obtained feature table?