Thanks for letting me know about this. The game attached to the bug report doesn't seem to be the one where the error occurred, but looking through your game history I've found a game where it might have done.
boardgamearena.com/table?table=582415678
Is this the game where it occurred?
Thanks for your assistance.

If so, and if the event occurred right at the end of the game, there is an explanation.

You did the "SOW + WATER OR FEED" action. When you do that, you MUST water or feed after sowing. If you had sowed the Melon, you would have no affordable water or feed actions (you had 3 food and 3 water, and Melon requires 5 food or 5 water, and none of your other sown crops had affordable actions either). So the other crops (Pumpkin and Pepper) were the only legal options because only they give you an affordable feed/water action.

If you had already had a sown crop with an affordable feed/water action, you would have been able to sow the Melon because you would have then been able to feed/water the other crop.

I'll mark this notabug assuming this is the case. I've added some information to the 'How to Play' section to make the rule a bit clearer, and used this as an example.