POP Pt toner bleaching effect

Would anyone have any idea why POP Pt toner would have almost a bleaching effect on VDB sensitizer? I have toned many VDB prints, with this toner, with the desired effect, however a couple of times this lightening effect has occured.

I just finished a VDB print and toned, after clearing and before fixing, using Pt toner. At about 1 minute the outside edges,where the brushmarks are, started to lighten up as it was taking up the toner. I have had this happen before a couple of times.

Any ideas on this?

Thanks.