You likely have multiple Streamlit installations on your machine. To confirm this is the case, display the Streamlit version within the same app:
import streamlit as st
with st.echo():
st.write(f"streamlit version: {st.__version__}")
tab1, tab2 = st.tabs(["First", "Second"])
with tab1:
st.write("First tab")
with tab2:
st.write("Second tab")
If you could share a screenshot of the entire app with the above code, Iād wager the displayed version is not 1.12.0.