Sorry, I can't reproduce the problem.
Whenever you have a problem, please post a complet test script, so the people who want to help you can re-create the problem and test their ideas.
Include a little sample data (CREATE TABLE and INSERT statements, relevant columns only) from all tables involved
Also post the results you want from that data, and an explanation of how you get those results from that data, with specific examples.
Simplify the problem as much as possible. For example, if you're really need to print 250 lines but 1 is getting lost, post an example where you only want 4 or 5 lines, and 1 is getting lost.
Always say which version of Oracle you're using (for example, 18.104.22.168.0).
See the forum FAQ: https://forums.oracle.com/message/9362002
Dbms_ouptut.put_line is a quick and dirty way to get output for debugging. If you're using it for any other purpose, change now, and the problem will become merely an acadeic question.
LINESIZE might cause an output line to wrap, or to get truncated, but it won't cause a line to disappear. If your LINESIZE is too short (than is, if you have lines with more than 3500 characters), then you might get more output lines than you expected, not fewer. If LINESIZE is longer than it needs to be, it won't change the output one way or another.
There's nothing special about the character '/' in dbms_output. '/' isn't causing the problem any more than 'B' or ',' or space.