This issue is fixed but the fix is not-quite-correctly attached to and LDEV-2029, which are resolved by a different fix. The issue itself was not documented, though.
In 5.2 (and 5.3 if the Ajaxproxy-doesn't-draw-JS issue is resolved) then the curly braces in AJAX calls using cfajaxproxy are not properly URL encoded.
https://github.com/lucee/Lucee/pull/593 resolves this for 5.2 and a separate PR will go out to the extension repo for 5.3, unless this stuff is remaining in the core in which case we'll submit it there.
Resolved in the extension repo by this PR: https://github.com/lucee/extension-ajax/pull/3