Running 5.1.5 as well and taxonomy fields are not loading for me either. Like RichB, I have multiple taxonomy fields in the edit post area–with 4 ACF fields covering 3 custom taxonomies and the built-in category taxonomy.
The taxonomies all have data as seen in the database and quick editor, but they’re showing as empty within edit post and saving the post with them empty clears the taxonomy values that did exist.
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