light resin film between models at mid height

[fbtn40]

Hello, using MARS 2 PRO, i have some issue when printing several miniatures at once. Since few weeks, i have a light resin film between models at around mid-height, just like if the whole screen flashes once at mid height of printing... Did not find any setting that could set this and looking file on chitubox reveals nothing.
Models are at around 2 or 3 cms each from others, currently using water resin setting, but did also with normal resin.

ty