In Vanilla, blocks that have item icon overrides (such as doors, cauldrons, and flowers) use that icon for command auto-completions involving blocks. Custom blocks with item overrides using custom items do not behave this way.

Demonstration

Below shows how cauldrons use their item icons — not auto-generated block icons — in auto-completions for the Block command enumeration.

Here is a custom block whose minecraft:icon references the same cauldron texture:

It incorrectly uses the auto-generated block icon instead.

Reproduction

This example is taken from the behavior pack in the following world attached for convenience:

This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such.