Im not to sure about filler. These are split for the book match grain. Im betting there was a crack prior to being glued together and mounted to the rim. Im also betting it was not detected at the time. But the crack was barely noticeable to detect at the refinish , assembly and QC prior to boxing up. I bet it started as a very fine slice. Then proceeded to get longer.
I used to have that issue with Brazilian In the raw it looked real Nice. At the time of the finish , it looked really Nice. After going through the drying process when the clear dried up. The barely seen cracks started showing through. Some through the sap wood , a few in the bookmatched quartersawn grains