The reason is probably this sample ''Slavic_Medieval:R3918''. It has a high amount of Iberian and Italic mix. I'll fix it.
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Distance: 2.1607% / 0.02160740
55.8 Slavic_Medieval
20.8 Byzantine
7.6 Balkan_Illyria
6.4 Pre_Slavic_Balkan
4.0 Germanic_Medieval
3.6 Celtic_Medieval
1.4 Baltic_Medieval
0.4 Iberian_Medieval
i removed your pre-slavic and balkan_illyrian and add different illyrian sample from Croatia (get better distance)
Distance: 1.9406% / 0.01940605
40.0 Slavic_Medieval
34.0 Illyrian
16.8 Byzantine
9.2 Baltic_Medieval
0.04985156 Illyrian:J-L283_Cetina_Croatia
0.05393507 Slavic_Medieval:R3906.SG
0.05419215 Slavic_Medieval:R6737.SG
0.05644887 Slavic_Medieval:I10430
0.05776325 Slavic_Medieval:R9662.SG
0.06191405 Slavic_Medieval:R3918.SG
0.06249998 French_Medieval:R11553.SG
0.06569840 Balkan_Illyria:R6693.SG
0.06646128 Celtic_Medieval:R11552.SG
0.07346956 Germanic_Medieval:DRU013.A_noUDG
0.07382240 Italian_Medieval:R1287.SG
0.07443537 Pre_Slavic_Balkan:R6730.SG
0.07565586 Germanic_Medieval:DRU007.A_noUDG
0.07853543 Germanic_Medieval:DRU001.A_noUDG
0.07888090 Iberian_Medieval:I12515
0.07998086 Iberian_Medieval:I12514
0.08302105 French_Medieval:R11561.SG
0.08316649 Italian_Medieval:VK534_noUDG.SG
0.08329948 Celtic_Medieval:KIL033.B_noUDG
0.08369681 Celtic_Medieval:KIL028.B_noUDG
0.08378973 Celtic_Medieval:KIL025.B_noUDG
0.08390129 Italian_Medieval:R1285.SG
0.08534364 Byzantine:I14821
0.09223247 Germanic_Medieval:DRU012.A_noUDG
0.09243653 Italian_Medieval:R1548.SG