I made another strange observation.
it seems that the property/field that is sometimes returned as single, is the one from the last edit.
I know made two query just for this two fields, as both have separate uids.
and both querys returned me the value of this field. so before and after the update.
now I fixed the code, to always add the uids on update.
I made 1 update. and the problem is gone.
now i only receive a single value all the time.
but interesting is that the 2 query for the fields itself now both return the same value.
even the uids are different, and even this value was not changed.