Something like that would indicate that the casino system probably detected something worth a manual check. In this regard, I imagine the casino informed you about the investigation and associated account blockage. Technically, it makes sense.
Sadly, in such situations, casinos do not usually share additional details with players, and until the check is done, there is quite an open space for speculation.
By the way, if the group share one system or platform, such check are held across the whole casino group, so you don't have to play in more than one. The system automatically flags certain player's account details across the whole group of casinos.
I understand it feels unconfortable from your point of view. And yet, until the investigation brings results, we may only guess whether it was truly necessary.
Regarding the bonuses, I'd like to say that casinos, like any other commercial subjects, are free to restrict anyone from receiving special offers and bonuses, since it represents a significant investment. Casinos and sportsbooks prefer to reward players who may consider leaving the site, or fresh new players seeking "motivation."
Find out more in this free e-course focused on bonuses 😉
Quoting:
"Let’s start with the basics, in case this is the first time you are learning about casino bonuses. Casino bonuses usually take the form of bonus funds or free spins, which are given to players for one of many reasons, including:
• As a reward for making a casino account
• As a reward for making a deposit
• As a reward for being an active player
• As an incentive to start playing again after a period of inactivity"
It may sound quite harsh, but still, it's a business focused on gaining income.
In any case, I hope the security check will be done quite soon and you will be able to withdraw the winnings. Keep us posted, please!