Ruby

 -P disable ruby code evaluation for lines beginning with % 

"lines beginning with %" means lines beginning with %. Your lines start with < or y, so -P is irrelevant to your input.

Oh sorry, I guess that's why it shouldn't change the output lol. will take a look

So the trick is:

$ erb -x repro.erb #coding:UTF-8 _erbout = +''; # ; _erbout.<< "x\n".freeze ; _erbout.<< "y\n".freeze ; _erbout 

$ erb -x -P repro.erb #coding:UTF-8 _erbout = +''; # ; _erbout.<< "x\ny\n".freeze ; _erbout 

At a glance, the generated code seems broken. However, @xtkoba made the point; <%# is a special token of ERB that allows a Ruby comment, whereas <% # is a Ruby expression with a comment. While it sounds strange that it's impacted by -P, whether -P or not, writing a comment in the middle of <% %> is currently not supported. For your case, you should just use <%# instead of <% #.