I hadn't done an upgrade but a fresh install; (v25.5.1 for Raspberry Pi 3 Model B running Armbian Linux 6.12.28-current-bcm2711)
Realised that I had the same problem, only channel 2 was present; though I only realised something was missing when I'd tried RaspberryOS on a Pi Zero with the same i2c device.
Adding to the /boot/config.txt didn't of itself solve the issue; adding the i2c1 to the armbian-config overlays also didn't see.
However I did find the `/boot/firmware/config.txt` that has a lot more settings, and seems to be where armbian-config edits
Note the `dtparam-i2c_arm=on` line... which was commented out. Adding that back and it works...
(well after figuring out I needed to level shift the i2c device but that's another story)
# For more options and information see
# http://rptl.io/configtxt
# Some settings may impact device functionality. See link above for details
# Uncomment some or all of these to enable the optional hardware interfaces
dtparam=i2c_arm=on
#dtparam=i2s=on
#dtparam=spi=on
# ..
dtoverlay=dwc2 i2c0 i2c1 i2c-gpio vc4-kms-v3d
