As far as I am aware the ORDLO prisoners are being treated, quite rightly so, as Ukraine citizens since DPR\LNR are not recognised internationally. Mainly they are being charged under 4 articles of the Ukraine criminal code and sentenced to 10-15 years in jail. Many of Zolkin's interviews talk about this and the specific criminal articles that they were charged with.
However the Ukrainians are still doing prisoner swaps of them to get their own soldiers back