I've only ever coded in js/ts; however, as I seek to grow my skills in react-native I've been trying to learn swift to deal with bridging. However, after coding a few hours in swift when I swapped back its like my brain stopped working and simple stuff I couldn't remember the syntax. After working in it again it kinda went back to normal; but, is this something that is normal or goes away with practice?
It goes away with practice.
What might help is to follow a script which gets you re-oriented with the codebase and gets you back in the flow of the module/language.
For example, I'd have stashed changes that add log lines to some logic, to "make it my own". Then I'd run the code and verify that I still understand the flow and ensure that I can make basic changes. Sometimes just getting the dev environment setup is a huge confidence boost.