I use Windows and when I opened your file had the same issue. Until someone more knowledgeable pops in - I suspect it's the line height issue - if you notice the ? that is beside the line height is highlighted, so might be a line height issue (which is why the text in your bottom text box doeesn't have space).
I ensured the ? was NOT selected, then I created a text box with 10 hz and it typed perfectly. For me, the arial seemed to go wonky moreso than say times new roman (no idea why). If times new roman works, as a workaround type using another font, then you can go back after typing, select all the text, then apply arial.