Because of the last posts in this tread and the 3rd software version of SFOS 4 for Xperia 10II, I tested my t-mobile Germany SIM card again in the XperiaX.
The result:
In SFOS 4.1.0.24, 4.2.0.19 and 4.3.0.12 the SIM card works perfectly in the XperiaX. When a call is made, the network switches from 4G to 2G and after the call back to 4G without delay and without interrupting the mobile network.
The same SIM card shows the bug described above in the Xperia 10 II (aarch64) in all three SFOS versions.
Note: The 3G network of t-mobile Germany has been switched off since 1 July 2021.