@WheelieNick has an excellent observation for one possibility of why the battery is not charged to 100% (assuming you left it charging while the laptop was turned off).
Other things to look at:
Make sure to connect the charger directly to the laptop since adapters, docks, hubs may not allow the full power of the charger to reach the laptop. Also try connecting the charger to another USB-C port especially on the other side of the laptop. Even rotating the USB-C connector of the charging cable 180 degrees upside down can make a difference if the USB-C port is half bad.
Unplug the charger from the electrical outlet for a minute to reset the charger's internal circuitry. Also make sure to try connecting the charger to another electrical outlet in case there is an issue with that outlet. If you were using a power/surge strip, then it may be bad. Here is an Apple article for troubleshooting USB-C charging issues:
If your USB-C power adapter isn't charging your Mac laptop - Apple Support
Try an SMC Reset.
Disconnect all other external devices besides the charger in case one of them is causing a problem.
I'm assuming you replaced the battery yourself with a third party battery?
There are a lot of things which can be causing this. Unfortunately these Apple USB-C laptops are very hard to troubleshoot and they have some very odd quirks related to power & charging. Having installed a third party Lithium battery now complicates matters since the quality of third party Lithium batteries is extremely poor even when purchased from a respected vendor such as OWC or iFixIt. At least before installing a third party battery you could have run the Apple Diagnostics to see whether any hardware issues were reported. Unfortunately the diagnostics many times will fail when a third party battery is installed even when the third party battery is completely healthy since some third party batteries don't communicate as the diagnostics expect/require. I guess you can try running the diagnostics, but the results are less useful now.
You could have a bad electrical outlet, a bad charger, bad charging cable, bad USB-C port(s) on the laptop, a bad or damaged Logic Board, or a bad battery. Or maybe you have an issue with an externally connected device, or perhaps a software/configuration issue. Or perhaps a combination of two or more of any of these.
A very important piece of information which you did not mention is whether the battery icon on the menu bar shows a lightning bolt charging icon.
