How can a person coming from a sanctioned country invest for their retirement abroad?

I realize this may be a difficult question without further details, but I'm hoping someone with experience in cross-border investing or changing residency status might be able to share some insights.

How are long-term investment accounts typically affected when an investor's legal residence status changes? For example, if someone is investing through a brokerage in their country of residence and later loses the right to reside there (due to a job change, visa expiration, relocation, etc.), what usually happens to their investments?

Are investors generally allowed to keep existing positions and simply stop making new contributions, or can accounts be restricted, frozen, or closed? How do brokerages typically handle situations where a person's nationality, residency, or sanctions-related compliance status creates additional scrutiny?

I'm particularly interested in understanding what happens in the long term, after many years of regular investing, in situations where the investor comes from a sanctioned country and invests small portions of their salary throughout the years.

Any experiences or resources would be greatly appreciated.