Previously placed fence posts are becoming unconnected. If they are removed and replaced, they will then connect with surrounding posts, and it seems to have happened in chunks, as some sections remain normal. Mobs treat the fences as normal and cannot pass between posts but player can.
Tried in multiple biomes, with several different types of fence, all wood, and will test non wood fences and walls going forward.
We're tracking this issue at MCPE-239562, so I will resolve and link this report as a duplicate.
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