iPhone's battery limit setting not working

It is supposed to charge to 100% on occasion in order to optimize the readings you see for your battery. If you are preventing that from happening it is going to keep trying. Apple documents that how it is supposed to work here:

• If you have Charge Limit set to less than 100 percent, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates.

https://support.apple.com/en-us/108055

The battery will charge to 100% "occasionally" to aid in proper performance calibration. This is normal. Here is an excerpt from Apple's Support Document:

Reference: https://support.apple.com/en-us/108055

Note that "occasionally" can mean different things to different people. Some users report that "occasionally" is several times a month, others report a longer interval.

When I tried Charge Limits, "occasionally" was 2-3 times a month. Your mileage will likely vary.