Why does changing a list in a record copy change the original record too?

The synthesized copy used by a record with expression is shallow for reference-valued members. Both records can still refer to the same mutable list. Use immutable collections for value-like data, or explicitly copy the mutable member when creating an independent snapshot. Decide whether element objects also need cloning; copying only the list structure leaves its reference elements shared. Do not advertise deep immutability from init-only properties alone. Create a copy, append to its list and check the original. Then test mutation of an element to reveal the second ownership level. Record syntax supplies conveniences, not a universal deep-copy or graph-isolation policy.