This has happened to me multiple times, most recently for example, Google's recent 20-1 stock split.
I held Google shares and had a specific profit on that. After the stock split, IG counted it as a "sale of all my shares for $0" and a "purchase of all the new shares for $0".
This is clearly a problem, because now that I have "bought a bunch of google shares for $0", my profit calculation for my overall account is way off. IG should either ignore profit on these shares or figure out a way to correctly determine profit.