IIRC, we first noticed this behaviour over 25 years ago!
Although the FMAs have changed to CLB/OP-CLB, the A/THR does not change instantly from SPD mode to thrust (CLB) mode. This is probably to give a smoother transition from ALT to OP-CLB (less G) for the benefit of the pax and standing cabin crew.
As SHUNT says, if necessary you can get the engines to accelerate immediately to climb N1 by selecting a higher IAS than the current IAS, and then reducing it. Or, if you're not worried about pax comfort, you can go for EXP-CLB temporarily (if still fitted?).
Enjoy!