If you have text right above or below the table, the preview still renders the table markdown as a table. But after you save the tablepost, the table is not rendered and just shows the markdown code.
You can use the code below as an example. If you try to edit my answer, it shows a table in the preview but not in the savedbackend-rendered post view.
Raw Markdown:
text above table |test|test| |----|----| |test|test|
How it looks in Preview
Result after posting
text above table |test|test| |----|----| |test|test|
So either the backend renderingrenderer is broken or the preview renderingrenderer is broken.