Well... we'll see. Certainly seems like a weird issue to me. What I found on the internet says AMD changed something about the Athlon 2xxGE series' pin assignment compared to standard AM4 and this causes the issues.
Since my original post I've received an answer from Biostar saying that there are two "flavors" of the CPU on the market - "flavor 1" will work after a BIOS update, but "flavor 2" won't. They also claim these two variants have the same OPN number so they are unable to tell which one works and which doesn't... So I've asked AMD's support for help.
The motherboard is already delivered to my friend's place but I only meet him in a month which is when I'll get the board. I'll buy the CPU at that time and am hoping AMD can help me with serial numbers to look for or avoid.
PS: just to make it sound less crazy with this motherboard, mini-ITX boards for AM4 are stupid expensive and I was lucky to find this one at a discount in a different country as an outgoing model, which is why I've ordered it. The discount practically pays for the CPU...