Currently, the Arrays extension isn't actively maintained and poses some weird behavior, as well as questionable implementation in some cases. Considering this, we should collect all issues in one place and decide what should be done about that afterwards.
Additionally I'd like to ask: Does anyone still use this CompatibilityMode feature introduced seven years ago? There are people thinking about removing that functionality.
As I consider Flow even worse than wikitext for dicussions and really confusing as well, I would like to discuss this topic on Phabricator instead. This will tracking the issues identified there easier as well. However, if you have any trouble in setting up OAuth for Phabricator, feel free to speak ub in this thread as well.