This is an intended behaviour, because you're technically just putting water sources in blocks where the glass panes in. It doesn't care which direction you're putting them in.
Also, this is a duplicate of "MC-127237 - Water can go through glass panes". Please use the search function in the future to see if the bugs have already been reported before.
They did fix the stairs to keep water from flowing out the back, so I do not see why they cannot fix this as well. And I did use the search function before posting. The search function did not pull up that issue. Thank you for your information, but it is unnecessary.
MC-133177
This is an intended behaviour, because you're technically just putting water sources in blocks where the glass panes in. It doesn't care which direction you're putting them in.
Also, this is a duplicate of "MC-127237 - Water can go through glass panes". Please use the search function in the future to see if the bugs have already been reported before.