my macbook has been experiencing a strange charging issue

Try an SMC Reset.

Make sure to connect the charger directly to the laptop since a multi-port adapter, dock, or hub may not allow the full power of the charger to reach the laptop which could cause this issue. Also disconnect all external devices.

Try unplugging the charger from the electrical outlet for a minute to reset the charger's internal circuitry.

Make sure to try the other USB-C ports as well especially on the other side of the laptop if you have the four port model. Also try rotating the connector of the charging cable 180 degrees upside down. I know this sounds crazy, but it can work if the port is half bad.

If the laptop is working hard with the CPU at 100%, then it can cause the laptop not to charge the battery.

While you could have a bad charger, it is much more likely to have a bad battery as suggested by @leroydouglas or to have an issue with the Logic Board or possibly the I/O Board if you have the four port model. These USB-C Apple laptops have a lot of issues and a lot of ways that can cause the charging circuitry to malfunction.

CJS1993 wrote:

I've owned this macbook pro for 5 years, and I have used the same USB-C power cable adapter for all 5 years. Recently my macbook wasnt charging because the USB-C cable was fried, so I replaced the cable and not the power adapter. Now it does charge again for my macbook, but with a catch: when my macbook isn't on, it charges without any issues and to 100% capacity; but when it is on, it doesn't charge fully and sometimes it fluctuates between charging up and not charging (that is, for example, it goes from 50% to 52% and then suddenly loses power and drops to 49% or lower). What could be the issue here? Should I replace the power cable or should I get my power cable outlet on my macbook pro checked up?

See if you qualify:

13-inch MacBook Pro (non Touch Bar) Battery Replacement Program- October 2016 and October 2017

https://support.apple.com/13inch-macbookpro-battery-replacement

15-inch MacBook Pro Battery Recall Program- September 2015 and February 2017

https://support.apple.com/15-inch-macbook-pro-battery-recall