Disappearing supply units

Supply units keep disappearing/being auto dismantled 2 or 3 game months after being created, when there are more than enough unemployed low level workers and medium managers (and no shortage of money). I know that I've not dreamed them because I see clearly in the "total size of army" graph the jump from, say, 18300 to 19320 then after 3 stable months fall back to the preceding level. Could it be fixed or at least explained? Latest example is Commonwealth of Roxan, White Giant. Thanks.