I even looked at the code for the version that was the current one in March 2021.
The call to generate a seed phrase is here:
https://github.com/Emurgo/yoroi-mobile/blob/4.0.0/src/components/WalletInit/CreateWallet/CreateWalletScreen.js#L72
generateAdaMnemonic is defined here:
https://github.com/Emurgo/yoroi-mobile/blob/4.0.0/src/crypto/byron/util.js#L155-L156
And the strength/length of the mnemonic is hard-coded to 160 bits in CONFIG.MNEMONIC_STRENGTH here:
https://github.com/Emurgo/yoroi-mobile/blob/4.0.0/src/config/config.js#L140
160 bits are a 15 word seed phrase:
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#Generating_the_mnemonic
It always has been that way for Yoroi. The code was moved a bit, but it’s basically still the same logic: hard-coded to 15 words and 15 words only. And they even let you verify it, so a glitch, where only 12 of the 15 were visible in the interface, is also unlikely.
Do you have another wallet app for Bitcoin or another blockchain? They have 12 words very often.