As I said, I only see this behavior on one single machine (customer laptop). When I run this on my own laptop with the exact same version of PLSQL Developer (15.0.4.2064) I can go up to the 32.767 limit of the varchar2 input parameter of put_line without any problems. (See screenshot).
If this is due to a limit in PLSQL Developer, I would expect to hit this problem on any installation of (this version of) PLSQL Developer. Or am I missing something?