That’s really, really not possible. Last time someone claimed to only have gotten 12 words from Yoroi, I even dived into the code: Yoroi 12-word seed (again!) - #4 by HeptaSean
They had 15 words hard-coded forever. No possibility that they gave out 12-word phrases.
If you never used Daedalus, then maybe some wallet for a different cryptocurrency? The Bitcoiners tend to use 12-word phrases. Is there any possibility that you mixed up the seed phrases of Yoroi and another wallet app?