Maybe there's also a time condition for Shoni? I always thought that the game would allow you to mooch even if the conditions for the moochable fish are not met and the failure would happen only at bite time.
Check ff14angler, their app has registered a successful Shoni catch after the Mahar mooch. Also, looking at Mahar's records list it seems like there were multiple HQ catches as the size differences between the first and the fifth places are insignificant:
1 195.7 unsung angler
2 194.6 発酵なまもの
3 194.0 unsung angler
4 193.7 テティス@Ramuh
5 192.6 はまち
