There is not a hole in the system. It is not a bug. It is not realistically feasible to completely prevent sock puppeting. To be clear, this behavior does violate Stack Exchange rules. Creating a sock puppet to evade your suspension and post while you are suspended is a clear violation of Stack Exchange rules. However, it is difficult to proactively prevent someone from doing this. We rely on users to report instances of ban evasion. Enforcement is based on detection and reaction rather than prevention.
If you see future instances of this, please flag it for moderator action, and provide as much evidence of the connection as possible.
Some tips to help you provide evidence of sock puppeting: From what I have seen, this user is repeatedly creating new accounts to evade their suspension. The new accounts are often listed under usernames like Mdayq or Mdayq7. Sometimes the user explicitly reveals the connection in their post or comments. It might be useful to know that this user goes by user49915 at Latex.org. Moderators have tools to investigate alleged sock puppeting, but the more evidence you can provide them, the more helpful your flag will be and the more likely that they can make a good informed decision.
Be prepared that moderators will have their own criteria for whether to act or not. Many moderators might act only if they are certain that this is a case of sock puppetry, as the consequences for getting this wrong are not good.
Some tips for moderators: If you find evidence of ban evasion, I recommend unceremoniously deleting the sock puppet account with the reason "This user was created to circumvent system or moderator imposed restrictions and continues to contribute poorly".
Separately: This user often cross-posts the same question on both Stack Exchange and on an external site. We don't have a policy forbidding that on Stack Exchange, but if you hold the personal opinion that this is detrimental to the site, you are free to downvote such questions; you always have that right, and it may send a signal over time.