Some alerts for Chromaggus don't work when he casts breaths, I narrowed it down to Frost Burn breath and found an incorrect spell id is in the list of conditions, this can be easily verified on wowhead or by fighting Chromaggus to confirm.

FIle: DBM-Raids-Vanilla\BWL\Chromaggus.lua

About line 196 you will see this code block:
function mod:SPELL_CAST_START(args)
if args:IsSpell(23308, 23309, 23313, 23314, 23197, 23189, 23315, 23316, 23310, 23312) then
warnBreath:Show(args.spellName)
timerBreath:Start(2, args.spellName)
timerBreath:UpdateIcon(args.spellId)
timerBreathCD:Start(60, args.spellName)
timerBreathCD:UpdateIcon(args.spellId)
end
end

Fix:
Update the "if" line, replace "23197" with "23187". Spell 23197 is the wrong ID for Frost Burn breath.

Thanks for report btw.
BTW interesting note about bug, it didn't fully exist (even with wrong spellID) until recently because way classic era worked, it only used spell names until latest patch, so it converted wrong ID to right spell name still. I expect more of these bugs exist in classic btw, another was fixed with a trash warning not long ago as well. Keep an eye out for others that don't work by ID because ID is incorrect.