That would be a problem with the win / stalemate detection
What version of the game do you have? Steam / GOG / Larian Vault?
If Steam you can request access to the upcoming patch before it is officially released (
see here), though I don't know if it includes any fixes for this particular issue. If GOG, the 1.0.33 patch should be released soon (though, again, this wasn't specifically mentioned in the changelist).
Does this also happen in a multiplayer campaign?
If you go into the advanced options in the multiplayer lobby, you can try changing the options for victory conditions, but I don't think any of the 3 listed would come into play in the first 10 seconds.