This still happens for me in Minecraft 1.13 (the full release). I found treasure map #0 in a shipwreck, found the treasure and explored the full map. Afterwards on my way home I found treasure map #1 in a different shipwreck. Map #1 leads to the same spot as map #0, even though I fully explored the first map before I found the second one. I can provide further details if you want.
This still happens for me in Minecraft 1.13 (the full release). I found treasure map #0 in a shipwreck, found the treasure and explored the full map. Afterwards on my way home I found treasure map #1 in a different shipwreck. Map #1 leads to the same spot as map #0, even though I fully explored the first map before I found the second one. I can provide further details if you want.