It may be more convenient for you to ask this question on the other forum: SAP SQL Anywhere Forum
Preamble...
It sometimes happens that (a) the same query will perform faster or slower in different versions because of different behaviors by the query optimizer. In most cases a minor change is necessary to restore good performance; in some rare cases, the changes are difficult.
It also sometimes happens that (b) the same query will perform faster or slower under different operating conditions; e.g., client request (ISQL) versus inside a BEGIN block (stored procedure). Again, minor changes all that's required in most cases, sometimes otherwise.
Details...
In order to help, we will need more information. Two graphical plans with statistics for the same problematic query, both using V16, one captured in ISQL, and the other one captured inside the stored procedure, is the best way to pass on the information; ALL OTHER forms are inadequate, especially prose descriptions and edited snippets of code (that way lies frustration for all parties).
There are a number of articles about graphical plans on this blog.