Step to reproduce
Place 5 floating solid blocks blocks in a row.
Place 3 more solid blocks on top of the 5, leaving the ends of the bottom row uncovered.
On top of the middle solid block you placed in step (2), place a coral or coral fan.
Spawn a villager on one side of the row.
Spawn a zombie on the other side of the row.
Expected result
The zombie walks through the coral to reach the villager.
Observed result
The zombie walks onto the coral but gets stuck there, jumping around. (There's a chance its jumping will move it far enough off the coral to resume pathfinding, whereupon it might eventually attack the villager.)
I've placed dead yellow coral fans in my barn for decoration, but it's causing mobs such as chickens to get stuck. Some mobs freeze in place, others will jump repeatedly in the same spot. It seems certain non-full blocks such as corals and carpets can ruin their pathfinding AI.
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[Mod] OcelotOnesie
Thank you for your report!
We're tracking this issue in MCPE-95129, so this ticket is being resolved and linked 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 to see if the issue has already been mentioned.
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GoldenHelmet
Reopening so that we can track this issue separately since MCPE-95129 was fixed for other blocks.
The problem with corals is probably that they are treated by pathfinding as if they are solid blocks, so mobs assume they cannot walk through them, just like they assume they can walk on them (MCPE-114407).
GoldenHelmet
Mobs jump around inside coral/fans for the same reason that mobs will not jump onto a block with a coral/fan on top: they treat the coral/fan as a full-size motion-blocking block, akin to solid blocks like stone and transparent full blocks like glass and leaves. This means that when they search for a path to get to a destination they assume that they can move on top of the coral/fan and cannot move through it. As a result, when they come to a coral/fan while moving on a level surface, they first try to jump on top of it. After they fall through and land on the block below the coral/fan, they keep trying to jump on top of the coral/fan because their pathing target was on top of the coral. Visually it looks like bouncing inside of the coral because the client-side animation of lags behind the server-side mob movement. However, if you use a particle command in a repeat command block to track the server-side movement of the mob, then you can see that the server-side position goes up above the coral/fan and falls back to the surface of the block below with each visual bounce.
[media]
The repeated jumping is just another form of the same behavior mobs display when pushed out of their paths by players or other mobs, or if you place a block if front of a mob while it is moving. Mobs that are moving always try to reach the next block on their planned paths for a few seconds whenever they fail to reach it for any reason. Here is an example where I open a trapdoor while a zombie walks across it.
[media]