18 words? The "common" number of words in a seed mnemonic is usually 12 or 24. Does Coinomi only use 18 word seed mnemonics? or are you simply missing 6 words? 
Seeds can be any multiple of 3. Even just 3 will work, we often use this for our tests, for ease and speed. (Not recommended of course for real use, as 3 words can be bruteforced easily).
Coinomi simply implements BIP32/39/44 technology, and the default seed given on a new installation was an option of 18 or 24 words. Latest releases removed the 18 words option, and give out 24 word seeds.
Coinomi uses the open source upstream project https://iancoleman.io/bip39/ (which was forked to https://www.coinomi.com/recovery-phrase-tool.html but now we uploaded all our coins and functionality back upstream, as the original project also includes SegWit derivation paths.)
You can use those tools offline. You can see that one can select any multiple of 3 words to generate a new random seed.
The OP saved the "confirmation" screen, which comes after the screen that presents the new seed, and shows *scrambled* words, so the user can confirm his sequence.
Saving a screenshot is wrong anyway, and worse, saving this screenshot on the phone, unecrypted, voids the whole point. User should write down the words in a physical paper and keep it safe, away from online devices.
Unfortunately as mentioned above it is computationally unfeasible to brute force a 18+ word seed.
