The change from bulk to absorp is triggered by battery voltage.
The charger does know how much output current is required to maintain the target battery voltage.
If a load cuts in during absorp, the battery voltage would drop, thus the charger would increase the output current until the battery voltage reaches the absorp target.
Likewise if a load cuts out, the battery voltage would creep up.
Then the charger output current will be reduced until the battery voltage settled back to the target voltage.