When inside water, lava or powder snow, it's possible to see the surface of that liquid from below. However, for cauldrons, this surface is completely invisible, which makes no sense.
Fixed model json files are attached.
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Can confirm in 21w03a.
Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 21w07a.
Can confirm in 21w08b. Video attached.
Can confirm in 21w13a.
Can confirm in 21w14a.
Can confirm in 21w15a.
Can confirm in 21w16a.
Can confirm in 21w17a.
Can confirm in 1.17.
Can confirm in 1.17.1.

Can confirm still in 1.18 Pre-release 1.
Can confirm in 1.18.
Can confirm in 1.18.1.
Can confirm in 1.19.
Can confirm in 1.19.2.
Can confirm