Whether in the car or the app, you'll see two numbers like this: 48/48a. The first number is the current charge rate, and the second is the maximum rate available from the EVSE at this time. It is only valid while the car is actually plugged in and charging.
When your charge goes down to 24a, you'll see either 24/48a or 24/24a. The first indicates that the car has lowered the charge rate, the second indicates that the EVSE has lowered the charge rate.
The main reasons the car would lower the charge rate is overheating at the charge inlet, voltage fluctuations that indicate a bad connection somewhere in the circuit or a supply issue at the utility transformer, some kind internal charging failure, or a third-party app is interfering with the charging process.
The main reason your wall connector (the EVSE) would lower the charge rate would be overheating at the internal connections. It would also be a normal thing in load sharing setups where two wall connectors are sharing panel capacity. When a second car starts charging, it would lower the charge rate on the first car to keep the total draw under the set limit.
The first step in diagnosing these problems is to change the password on your Tesla account to lock out any third party apps or services that may have access to your car. We've seen too many times where people delete an app and think that's the end of it. However, the real functionality is not on your phone but on the app's servers. An app you think is doing nothing can suddenly spring to life and start doing things you don't expect.
So, no matter how sure you are that third party apps aren't the problem, do it anyway.
After that, regular troubleshooting techniques come into play. The simplest one is generally to try your car on other EVSEs and have other cars try yours. If you can get the problem to happen in different combinations, it will help narrow down the problem.
