Obviously it's against the spirit of the thing, but I don't think it's wrong per-se.
>Complain to whoever gave you the bad link, and ask them to stop modifying URLs, because it’s bad manners.
It's ironic that an error response so blatantly violating the robustness principle is throwing shade about bad manners.
In our modern world, the robustness principle has become an invitation to security bugs, and vendor lock-in. Edge cases snuck through one system on robustness, then trigger unfortunate behavior when they hit a different system. Two systems tried to do something reasonable on an ambiguous case, but did it differently, leading to software that works on one, failing to work on the other.
That said, we are paying a huge complexity cost due to our efforts to allow nonconforming pages. This complexity is widely abused by malicious actors. See, for instance, https://cheatsheetseries.owasp.org/cheatsheets/XSS_Filter_Ev... for ways in which attackers try to bypass security filters. A lot of it is only possible because of this unnecessary complexity.