OK so for those interested in the subject, @fab1An and I got to the root of this issue with Find and Replace plugin missing its vital bits of functionality out of the blue: it’s regular expressions that are causing unavoidable troubles.
I’ve published a fix as a PR on GitHub: Fix passing state containing regex with backslashes down to the webview by rodionovd · Pull Request #132 · thierryc/Sketch-Find-And-Replace · GitHub
(there’s also a link to download a fixed version of the plugin in there. Please use it until the plugin author merges the PR and releases a proper update).