I play on samsung galaxy s8+.
This bug affects all my worlds and happens every time when I switch to another app (for example, chrome, or any other) or just exit to home screen. So when I return to the game after this actions, all animations stop, including water, lava, portals, splash screen loading animation, etc. They look just hung up - the texture is proper, but it doesn't move.
I don't attach any screenshots since they won't be useful.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-39749.
Please add a vote and any new information to the main ticket, which will be updated accordingly.
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📧 Suggestions – 📖 Minecraft Wiki