I know about 6mm micro DD driver (kinera Tyr, Yamaha EPH-100, EPH-200...) but I don't know if there is a 3.8mm DD at all.Found the Fischer Audio IEM i was talking about, the Dreamcatcher, it does look really tiny, they only have a 3.8mm DD.
This model looks like EPH200 but I don't know if this copy has MMCX sockets
https://www.aliexpress.com/item/4001358794582.html