Actually no, but this is because we sacrificed the sin-theta range below .32000 as the crystal is too big to fully enter the objective port. This was to be able to get the background around UMb. Had we thought about Ti at that time, we could have cut just a little more off of the corner to get down to this wavelength. Next time!
If I recall, Dave Wark found these holes also on standard PET. There may be an orientational aspect to this too, depending on how the crystal is mounted on the holder. Part of why they show up on high count rate crystals is that they are high count rate crystals. You obviously have to have sufficient precision on the WDS scan to see these. This is part of the issue in trace element work is that evaluating the wavelength region at low precision relative to the actual peak counting precision can be really misleading. But of course it takes forever to really do these sorts of high precision scans...hence the use of blanks, the PfE blank correction, and multi-point scanning! Although, as you have shown, for a simple matrix like SiO2 the background may be done pretty well by MAN estimation.