The flower pot bottom texture is not rendered, but the "uv" is still determined in the JSON file.
Basically, the determined "uv" is looking at a transparent texture, leading to a unrendered bottom surface. To fix this, the "uv" should just be moved to a different place. A sample fixed JSON file is attached.
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This issue is a Duplicate of MC-68206.

@Bemoty
Yes, but it is slightly different to the duplicate report. Plus, according to the JSON file, this IS a bug. There should've been a bottom texture, however, the devs made a mistake when creating the model. Also, my report includes a fix. Please make MC-68206 a duplicate of this report.
I forward-resolved the old issue instead as it was quite outdated and this one is more detailed and includes a fix.
According to the model file, the bottom side of the flower pot should actually be rendered, but the UV values just are incorrect to include the transparent part of the texture.

@violine1101
Yes, that's exactly what I wanted to say.

Confirmed for 1.13.1.

Affects 18w47a and 18w47b

Affects 18w48a

Affects 18w48b, can I request ownership of the ticket?

No

ok
Affects 18w49a

Affects 18w50a

Affects 19w07a

Affects MC1.14.2.

Note that this also affects the model files flower_pot_cross.json and potted_cactus.json, which use the flower pot elements too.

this happens in 19w35a

Still in 19w38a/b

Affects 19w40a

Affects 19w41a

Affects 19w42a

Affects 19w44a

The bottom of potted bamboo, cacti, and ferns still have no under texture with your model
An easy fix, just go into the files for them and make it the same bottom texture you have with the other flowerpots, then add each new json here

Only fixed for empty pots, all others still lack the bottom texture. Please reopen!

I have create a new ticket MC-170935.