RAW is problematic on this point. I am therefore going to address this question in a rather unusual order for a rules-as-written question: I am going to discuss the (highly-likely) intent, and then discuss what the rules do or don’t say and why they are problematic.
The intent seems pretty clear: the masterwork benefit is included in mithral’s
In a very-unusual case, I believe we can safely determine the intent of how mithral should be handled. We have examples of specific items statted up in the Player’s Handbook, including the elven chain and mithral shirt. These appear (particularly in the latter case, since it doesn’t have the fancy name) to be by-the-book examples of the mithral special material being applied to the chainmail and chain shirt items, respectively.
In both cases, the armors are statted with the masterwork bonus not stacking with the benefit of mithral.
It is important to note that these examples do not provide rules for applying mithral. They are, at best, secondary sources for what it means to be mithral, and specific items can and do provide exceptions to general rules; it is entirely possible to have specific items differ from the items you would get by applying the modular pieces the item appears to be made of.
And even in particularly blatant cases like the mithral shirt, examples are not the primary source for the general rule. In a contradiction between the two, the errata rules would have us ignore the examples and follow the primary rule.
Thus, we can only make an argument for intent on the basis of these examples, but I argue that in this case, they provide a very strong argument for intent.
The rules fail to describe this, however
Because the examples above give a very strong indication of the likely intent of how mithral should be applied, we expect the mithral rules themselves (the primary source, now), to describe the process such that we get the results indicated by the examples, where the benefit of mithral supersedes or includes that of masterwork without stacking.
In particular, one of these would suffice to get the results we expect:
A statement that says that the benefits described for making an armor from mithral already include the benefit for being masterwork.
A statement that says that these two benefits should not stack with one another.
However, the rules say only that the cost of being masterwork is included in the cost of being mithral; they do not say whether or not the benefit of being masterwork is included in the benefit of being mithral. That’s one option down.
Furthermore, the general stacking rules are no help, even if we accept that the reduction of ACP is a bonus subject to the bonus-stacking rules,1 since the benefit from both masterwork and mithral are untyped, and they are separate sources. Under the usual stacking rules, these two benefits would stack.
Super-strict RAW, two separate benefits are described, they are not given types, and there is nothing in the rules that says they should not stack. However, available examples strongly suggest that this was an oversight, to the point that I would consider it dishonest to fail to mention them.
- The bonus stacking rules define a bonus as a number added to a d20 roll; a reduction in a stat like ACP doesn’t really qualify. However, the rules make a frequent habit of treating other things as bonuses despite that definition, and I suggest it makes sense to do so here.