I'm using a solid pine door.
Somehow it worked now.
Changed the glass slabs to full glass block, didn't help.
Extended the 7x7 dirt platform under the farm land to 9x9 to cover the wall under the water block, didn't help.
Further extending the platform out and up 1 block, so the outer facing sides of the water block are all covered. Suddenly, greenhouse.
In short, don't build greenhouse on a lake, just build it on land like a normal person would.
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Charles Tang's post in Greenhouse not working? was marked as the answer