where to permanently set console output

Since upgrading to 14-stable, I have found that when viewing through
the serial console (rpi2b-provided), in the beastie menu,

5. Cons

resets itself to Dual (Video primary), every reboot, regardless of
what I then set it to.

The behaviour before the upgrade was once it was set to Serial,
it remained at Serial over multiple reboots. What do I need to
change to make it stay Serial?

This is a rpi4b running headless, stable/14-n265566

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