Not A Bug Doubling Cube gives generic mana from Mishra's Workshop instead of "only artifact spells" mana. Similar error on overabundance (maybe others)


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This is not a bug; it is working as intended.
See Doubling Cube's page on Gatherer:
Any restrictions on the mana in your mana pool aren't copied. For example, if you have {C}{W}{W}{B} with no restrictions on it in your mana pool and {U}{U}{U} that can be used only to cast artifact spells, you'll end up with {C}{C}{W}{W}{W}{W}{B}{B}. {U}{U}{U} that can be used only to cast artifact spells, and {U}{U}{U} that can be used for anything.


Confirmed from Comprehensive Rules:

106.6. Some spells or abilities that produce mana restrict how that mana can be spent, have an additional effect that affects the spell or ability that mana is spent on, or create a delayed triggered ability (see rule 603.7a) that triggers when that mana is spent. This doesn’t affect the mana’s type.

Example: A player’s mana pool contains {R}{G} which can be spent only to cast creature spells. That player activates Doubling Cube’s ability, which reads “{3}, {T}: Double the amount of each type of unspent mana you have.” The player’s mana pool now has {R}{R}{G}{G} in it, {R}{G} of which can be spent on anything.