He's placing a bet across different platforms on the same game for the same result. Far as I know, what's prohibited is arbitrage betting, where the bettor placed bets for multiple outcome. Given OP placed bet for the exact same outcome, I don't think that's prohibited. Unless a clause in BC's ToS specifically prohibit this... can you perhaps point it out to us, please?
Odds providers usually limit the maximum bet amount a user can place on a market; this is a common risk management strategy implemented by every odds provider. When a user exceeds the limit placed, they're not allowed to place more on the same market, and since we share the same odds provider, the limit was enforced cross-site. The 5-day delay in voiding the bet makes it awkward, but this is what has transpired.