I have noticed that if you have the service date ON on your invoice then the blank lines indeed do get stripped away (and if you add text you have no control on where it appears as the lines get sorted by service date). I just add the service date in the memo now (in the note on Tsheets) and don't select the actual field on my invoices. This lets me add blank lines as well as text where I want them.

Account & Settings / Sales / Sales form contents / Service date : off