Could you give me some advice about dealing with these subhalos?
Recently I encountered some subhalos without progenitor and descendant. I am wondering why these subhalos have no merger tree and how to add some criteria to avoid them. Would a limit on mass help?
Yes it's true, I imagine most of these are very small objects near the limit of 20 particles (per subhalo).
If you enforce a limit of 1000 particles per subhalo most objects should have merger trees.
On other hand, if you just want to avoid these cases, you can check if they have a tree, and if not, skip it.