We are currently migrating from a Matrix v4.12 system to a Matrix v5.1 and I've come across an odd behaviour in Custom forms. In the Page Contents, after a form is submitted, the %form_errors% keyword replacement does not seem to be doing anything.
Newer keyword variants, such as %form_errors_message% and %question_error_xxx_qyy% are working fine - this is how I'm confirming there are actually submission errors.
Ordinarily I would use the error message on the individual question (one of the newer keywords and is working), however this is an old form that has been mirrored across and is quite complicated. I'd prefer to not have to re-write it. Manual indicates the keyword should be working.