Hello ,
I was able to reproduce this issue on my end. I have written up a report ( UE-8501) and I have submitted it to the developers for further consideration. I will provide updates with any pertinent information as it becomes available. Thank you for you time and information.
I do however, have a way to achieve what I believe is your goal. If you check both Manual word wrap and Single line you will then need to set each line as though it were on a timeline. For example if you would like to have each subtitle show every 1.5 seconds you would put their time in as 1.5, 3.0, 4.5, 6.0 7.5 ect. Here is a picture of what I mean:
I hope that this helps.
Make it a great day