In Vienna, the shop I bought my supplies had all products tested by local TÜV NORD and they mixed liquids on site. Nowadays I buy and mix myself from standard 50/50 PG/VG mix from known source. I had mixed results with imported premixed liquids (off smell, not consistent viscosity - went to bin) and that promptly stopped me from buying more of premixed stuff (along with problematic regulations of container sizes).

Anything you add to the liqiud other than nicotine and PG/VG may be problematic. You never really know what is in imported premixed liquid, what cutting agents were used, what purity of source material was.

It seems that CDC is seeing some results of cost cutting, using stuff that was not really meant to be vaped mixed to keep margins better.