WhatsApp says "official app needed"

I did a few trial and error testing, and I found out that an unlocked bootloader might impact it.
Basically, I revered an old Xperia X back to Android with Emma and installed Whatsapp, and still got the “official app” needed message. → bootloader unlocked.
I relocked the bootloader, and reinstalled Android and Whatsapp, and no error anymore!