I wouldn't say usually, but they can happen from time to time for a variety of reasons.

It can be caused by overly aggressive WAF (web application firewall) configurations, proxy server misconfigurations, bugs in Lemmy and probably some more.

Proxy server misconfiguration is a common one we've seen other instances have issues with from time to time, especially when it works between Lemmy instances but e.g. Mastodon -> Lemmy not working properly, as the proxy configuration would only be specifically matching Lemmys behavior rather than spec-compliant requests.

Overly aggressive WAF configurations tend to usually being a result of instances being attacked/overloaded either by DDoS or aggressive AI service crawlers.

Usually, when there are no configuration changes on either side, issues like this don't just show up randomly.

In this case, while there was a change on the lemmy.ml side and we don't believe a change on our side fell into the time this started happening (we don't have the exact date for when the underlying issue started happening), while the behavior on the sending side might have changed with the Lemmy update, and other instances might just randomly not be affected. We currently believe that this is likely just exposing an issue on our end that already existed prior to changes on lemmy.ml, except the specific logic was previously not used.