WMS layer content is rendered with a white backdrop

Hi Team, not sure this exact issue was already covered, but a few existing i reviewed have nothing to do with this strange behavior.

BDL the service of Polish Forestry Service recently updated what/how they serve the layer information. The URL to the WMS remained the same, but instead of solid polygons, now they present just the contours in a style that matches how their web/apk maps show that layer.

https://mapserver.bdl.lasy.gov.pl/ArcGIS/services/WMS_BDL_Mapa_turystyczna/MapServer/WMSServer (i've added it to Locus library some years ago, the only difference i see, is they now advertise it with https://)

While in their Web and APK maps the layer in particular (Obszar Zanocuj w Lesie - areas where it is allowed to spend a night or two) is showing correctly, Locus unfortunately has a wierd issue of adding opaque white background to that layer. Luckily we have the visibility slider, and with ~25%-50% it's usable, but whole base map looks like extra opacity filter is applied.

Initially i contacted them, to inquiry what changes they did which could cause this. That was over a week ago, with no reply (in other cases they were fast and responsive) but since them i tried playing around with those layers using a browser and it seems like some strange issue with PNG transparency on Locus instead. Could it be the case?

Attaching some screenshots + png saved directly from arcgis map viewer https://www.arcgis.com/apps/mapviewer/index.html but it's the same when vising BDL original geoportal: https://www.bdl.lasy.gov.pl/portal/mapy?t=0&ll=17.574198,50.910188&scale=72224&map=8,1.0&layers=0,1,2,3,4,5,6,7,8,9,10,13,14,15,16&basemap=1&extwms=&hist= (or direct PNG https://mapserver.bdl.lasy.gov.pl/ArcGIS/rest/services/Mapa_turystyczna/MapServer/export?dpi=96&transparent=true&format=png8&bbox=1942563.4364594803%2C6594558.707572808%2C1970138.0944133205%2C6616285.93286284&bboxSR=102100&imageSR=102100&size=1443%2C1137&layers=show%3A1%2C2%2C3%2C4%2C5%2C6%2C7%2C8%2C9%2C10%2C13%2C14%2C15%2C16&f=image)

+ rechecked using most recent beta (Friday 13th, no luck...)
+ attached wms service capabilities