The bug
The inner cube inside the honey block and the slime block isn't displayed in the inventory or when it's dropped on the ground. This bug is back after it was once fixed in 15w47a (MC-67017).
NOTE:
As of 19w45a, dropped slime and honey blocks display their inner cube again, but still lack it in the inventory.
Fixes
Proposed fixes by @unknown:
Linked issues
is duplicated by
relates to
Attachments
Comments

Still an issue in 19w38a/b

Affected in 19w39a.

Affects 19w40a

Affects 19w41a. Also happens with the new honey blocks!

I've attached model fix for Honey Block as well.

Affects 19w42a

Still not fixed in 19w44a

Partially fixed for 19w45a, dropped slime and honey blocks show the inner cube now. Still missing it in the inventory. Updated description and summary.
Also affect honey block from advancement menu.
How do you use the proposed fix?

The inner cube not showing in the inventory in MC-134970
Affects 20w46a
Affects 20w48a
Affects 20w49a
Affects 20w51a
still not fixed
Can confirm in 21w03a.
Can confirm in 21w06a.
Cannot confirm: the inner cube shown for me
I am still able to confirm this in 21w15a.

Confirmed for 1.17-rc1

Affects 1.17

Can confirm in 1.17.1 Release Candidate 1.

Affects 1.18-pre6

Affects 1.18-pre8

Affects 1.18
Can confirm in 1.18.1.
Can confirm in 1.18.2 and 22w16b.
Can confirm in 1.19.

Can confirm for 1.19.3

Can confirm for 23w03a

In 23w04a
Clone of MC-67017