First set the max health to an odd number by

/attribute @p minecraft:generic.max_health base set 23

Then give the absorption effect

/effect give @p minecraft:absorption 100000 2

The result is right: (12 hearts with 23 hp + 12 absorption)

But after taking a damage of 1 (half heart), there is a display bug:

Actual: 11 hearts with 22 hp + 11 absorption

Expected: 12 hearts with 23 hp + 11 absorption

If taking another damage, the heart will come back

(12 hearts with 23 hp + 10 absorption)