The texture of the light block was ported over from Bedrock, which uses a 32x32 resolution to convey a readable number. However, the block functions differently on Java Edition, in such a way that it doesn't require the light level to be displayed.
Linked issues
is duplicated by 1
Attachments
Comments 8
The block only has an additional cycling light level function, pick blocking a level 7 light will make you hold a level 7 light. So it definitely makes sense to me.
Most likely WAI/feature request, though it's not impossible for a number to be conveyed in a 16x16 texture.
Crude mockup:
Can confirm.