Thank you, everyone, for such valuable suggestions!
It indeed looks like external interference affected your solution. As Bryan suggested, you can check the models available online to ensure that the effect of the interference is at a minimum.
We have a support tip explaining the difference between NTRIP and your own base. With the same observation time, your accuracy shouldn’t be affected. But It’s important to set up the local base correctly. Another key factor is the baseline. NTRIP bases are often located at some distance from the site, and you can place your own base just next to the rover. That’s why the base and rover setups can provide higher accuracy in some cases.