Battlechip / Arena crystals still do give Units. Should be at a 15% chance per crystal opened.
Sometimes you just get unlucky and don’t get any Units from opening a set of 10 crystals (and might not even from several sets of 10, depending on how unlucky you are).
Units are still available in the Arena crystal, and there's no "reduction" in the amount of units. Units aren't exclusive to arena crystals, you can farm units via other means too.
Just pulled 225 Units from a 5 pack of UC Arena Crystals last night. They're in there. Sometimes you'll go through a drought where there's seemingly none available. That's just how RNG works.
They should return units to the event quest though. It's not even like it was very much. Considering the 3-6* featured crystal is basically a guarenteed 3* 99/100 crystals it's just another way to be stingy.
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Sometimes you just get unlucky and don’t get any Units from opening a set of 10 crystals (and might not even from several sets of 10, depending on how unlucky you are).