It is a common problem for all baritones: the diameter of the smaller bore, compared to the euphonium, exacerbates the accumulation of condensation, especially in cold environments. There is little to do: empty the condensation from the slides using the water keys instead of pulling them, it only takes a few seconds.
2007 Besson Prestige 2052, 3D+ K&G mouthpiece; JP373 baritone, 4B modified K&G mouthpiece; Bach 42GO trombone, T4C K&G mouthpiece; 1973 Besson New Standard 3 compensated valves, 3D+ K&G modified mouthpiece; Wessex French C tuba, 3D+ K&G modified mouthpiece.