MacPorts updated flang-19 to 19.1.7 on Jan. 14, 2025. It shows as building successfully on all 3 of the Arm64 buildbots:

https://ports.macports.org/port/flang-19/details/

I recommend updating your port tree, cleaning and try again to install.

Note that the project does not yet supply binaries for Sequoia/Arm64 so the build may take a long time.