@AndrewFFFF First of all: a series resistor for the LED is not necessary, as the CHRG pin supplies max 10mA.
The behaviour is interesting because the LED on pin CHRG of the STC4054 can only flash if the status of the IC changes periodically, e.g. if the supply voltage collapses for reasons to be investigated. Otherwise, the CHRG pin has exactly three possible states, which are described in the data sheet, section 6.6. In my opinion, the board with the flashing LED is defective, e.g. because there is a soldering problem, component fault or something else.
What you refer to as ‘less bright’ is probably the weak pull-down state, where a modern LED can easily light up at 20µA. In this case, the battery is fully charged and the STC4054 is in standby mode. This board is probably functional, which you can check by connecting a discharged battery (battery voltage between 2.9-4.05V), whereby the LED should light up at full brightness.
Please read AN2370 for a more detailed description.
Regards
/Peter
P.S. (you have inserted your schematics while I was writing my answer)
Please remove the capacitor between VCC and CHRG!
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