Looking at Daybreak Arrow, it talks about adding extra damage to undead, then includes,
This extra damage and half of any other damage you deal with an affected projectile results directly from radiant energy and is not subject to damage resistance
Does this mean that it bypasses DR for all creatures, or only undead? The Mythic version explicity states it only bypasses all DR for undead.
If we take this as it seems, this would be a great way of dealing with high DR monsters.
It looks like this thread got a response from the person who originally wrote the wording for Daybreak Arrow.
Hi, I'm Ben Bruck, I wrote the spell Daybreak Arrows for Ultimate Combat.
Your GM was unfortunately running it wrong in this case (or at least not the way I intended it when I wrote it). Half the damage dealt by the arrows is essentially energy damage, and should bypass DR, regardless of whether the creature is harmed by sunlight or not.
Additionally, as blahpers says, the arrows deal an additional 1d6 damage to all undead, not just undead harmed by bright light.
Extra words aside, yes, half of the damage done by the ammunition itself always ignores DR, regardless of enemy type.
Keep in mind that Ultimate Combat (The source of Daybreak Arrow) and Mythic Adventures (The source of Mythic Daybreak Arrow) were written at least a year apart (UC was 2011, MA was 2013). It's entirely possible that the Mythic version was written by a different author, who had a different interpretation of the original spell. Equally possible, the extra DR avoidance from the Mythic spell comes at the cost of the more narrow enemy type, and is intentional.
Regardless of why it happened, RAW (and it seems, RAI), you are able to bypass DR with half of your ammunition's damage with Daybreak Arrow, regardless of the creature type. Happy shooting!
