Yes, it happened many times, mainly with some low rate drops as stun charge, protector, repair target, taunt, etc...
Note that white missions for blueprint looting (most people say "white" instead of "sideline"...) give you a non mandatory location. You can move to other site with mantis of same type and level. In some planets even the same blueprint can be dropped by enemies of lower level. Per example in Aurora the avengers of rank 13 and 15 drop Ion Storm blaster, so you can hunt for it at center of map (r15) or at south-west (r13)...
If you tell us the blueprint that you are trying to get maybe we could point you to a good place. It can be also that some white missions mark a wrong place (it is suppossed that most of those bugs were already fixed, but...) Check that the mantis written in the text match the ones in target site.