Hi lih,
It seems their intention is to support IE11 https://github.com/nextcloud/browserslist-config/blob/master/browserlist.config.js
My guess is the problem is in one of the dependencies which doesn’t respect this and has no Proxy poly-fill.
Not sure if maintainers will address this so I would recommend installing the legacy Text app which needs to be done similar to the explanation here via command line: [SOLVED] Does Markdown Editor no longer work?
But to see if they will look into fixing this with standard Text app open an issue here: https://github.com/nextcloud/text/