I blame FileImporter, it mimics the normal editor too well, and tricks the 2017 wikitext editor into thinking that you're trying to edit a normal file description page.

We could make the 2017 wikitext editor keep the prefilled wikitext, but that wouldn't fix the issue – if you went through the editing and published, it would probably immediately create the file page (without a file) with your wikitext. More complicated changes would be needed to make it return to FileImporter.

This bug also occurs without 2017 wikitext editor, when only visual editor is enabled – the edit interface lets you switch to visual mode, which would immediately save the page instead of returning to FileImporter:

If FileImporter needs to mimic the editor so closely, then the fix is probably to explicitly disable VisualEditor on this page, similar to 3ba918a6aecafc0880ec82cbe5602804117477ad or a0af0b05299a3f9e96ace3cdf84cf83e40f5ea0e.

I did a bit of investigation, but couldn't find a quick and easy solution.

• It's indeed like @matmarex says, FileImporter "mimics the normal editor too well".
• This is necessary to enable the 2010 WikiEditor (2010) toolbar as well as MediaWiki-extensions-CodeMirror syntax highlighting.
• Making this less perfect would not only disable VE, but also the features we want.
• I tried a lot of things to somehow disable VE from FileImporter: Not load the VE RL module in the first place; force some user option to disable VE; trick the VE initialization into thinking it can't work; use a fake namespace where VE doesn't work; … These attempts all have the same issues: There is currently no way to do this. And even if, most ideas depend on a specific loading order.
• Somehow using an extension.json attribute to disable VE from another extension might be an option (example).

Hard-coding an extra check in VE – as suggested by @matmarex – is probably the best option for now.

Change 756599 had a related patch set uploaded (by WMDE-Fisch; author: WMDE-Fisch):

[mediawiki/extensions/VisualEditor@master] Allow disabling VE and 2017 wikitext editor from FileImporter

https://gerrit.wikimedia.org/r/756599

Change 756600 had a related patch set uploaded (by WMDE-Fisch; author: WMDE-Fisch):

[mediawiki/extensions/FileImporter@master] Disable VE and 2017 wikitext editor on Edit File Info

https://gerrit.wikimedia.org/r/756600

Change 756600 merged by jenkins-bot:

[mediawiki/extensions/FileImporter@master] Disable VE and 2017 wikitext editor on Edit File Info

https://gerrit.wikimedia.org/r/756600

Change 756599 merged by jenkins-bot:

[mediawiki/extensions/VisualEditor@master] Allow disabling VE and 2017 wikitext editor from FileImporter

https://gerrit.wikimedia.org/r/756599

Thanks, @WMDE-Fisch! This looks like it should work correctly now.