After upgrading to Debian 12, xmobar ignores the value for the -O
option in the battery monitor.  From my config,

      Run BatteryN ["BAT0"]
              ["-t", "<left><acstatus>",
               "-L", "30", "-H", "74",
               "-l", "red", "-n", "orange", "-h", "yellow",
               "-S", "on",
               "--", "-O", "<fc=green>▲</fc>", "-i", "<fc=cyan>=</fc>",
               "-o", "<fc=yellow>▼</fc>"]
              30 "bat0",

This worked for several years, in that "▲" would appear while the
battery is charging but now it's blank.  The strings for the other
states ("=" and "▼" in my case) do still appear as expected.

Please let me know if I can provide more information.
Thanks