Here are the steps to reproduce the glitch:
Place inverted stairs or slabs [As shown in images]
Place water inside them and put slabs so it doesn't flow (It doesnt matter if it flows) [As shown in images]
And mobs wont be able to jump over them, flying (Ghasts,blazes) jumping (magma slimes, slimes) and climbing (spider, cavespiders) mobs will still be able to jump over these blocks.
Thank you for your report!
We're actually already tracking this issue in MC-127149, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki