Function calls in ksh (and in other POSIX conforming shells) do not cause a while loop that invokes that function to terminate unless the function does not return when it terminates. If you would show us the code for your process_line function and how the variables that are used by the code you showed us are initialized we would have a much better chance of being able to help you.

There are several places in your code where you expand shell variables without quoting the expansion. Depending on the value of the variables involved, that may send output to a different file than you intended and cause syntax errors.

The awk substr() function's 2nd argument is the starting position within the string named by the 1st argument. You are using 0 for that argument, but the 1st character of a string in awk is number 1; not number 0. (And there are much more efficient ways to grab the 1st 8 characters of a string contained in a shell variable than invoking a subshell, creating a pipeline of two more processes and invoking awk.)

With what you are doing with read in your while loop, there doesn't seem to be any obvious reason to change IFS from its default value for the read. What do you hope to achieve by setting IFS to a comma?

With what you have shown us so far, my 1st guess would be that process_line is exiting your entire shell script instead of returning (or falling through the end of your function) which would return control to your while loop. My 2nd guess would be what RudiC has already suggested. But without a lot more details than you have provided so far, we are all just guessing.