When looking at the different scenarios of how TermboxView is invoked and what happens in error cases we observed an interesting behavior: TermboxView::renderTermbox() was invoked repeatedly (3 times) when running into SSR errors (i.e. no result written into ParserCache) while trying to display an item page.
Upon further investigation it was found that RefreshLinksJob & DerivedPageDataUpdater, i.e. jobs run after the connection to the client is already closed, are the source of the two extra calls.
Given that they do what they are designed to do and (actively or as a side effect) also ensure that ParserCache contains meaningful values this is expected behavior.

Risks:

• in case of problems generating a termbox result (through a service request) the expiration time for this page's cache will be set to 0, i.e. every following request (incl. but not limited to jobs) will also try to generate a termbox result by the same means until a result is found and successfully persisted in the ParserCache

Possible mitigations:

• implement some form of throttling (for that scenario in particular or any request in general)

Change 494717 had a related patch set uploaded (by Pablo Grass (WMDE); owner: Pablo Grass (WMDE)):
[mediawiki/extensions/Wikibase@master] OutputPageBeforeHTMLHookHandler: editablity as a service

https://gerrit.wikimedia.org/r/494717

Change 494717 merged by jenkins-bot:
[mediawiki/extensions/Wikibase@master] OutputPageBeforeHTMLHookHandler: editablity as a service

https://gerrit.wikimedia.org/r/494717

The languages shown on entity pages should now be identical for the new and the old termbox, even (edit: found an unrelated inconsistency: T218111) when server-side rendered. Steps to verify:

• log in on beta
• add languages to your Babel box e.g. {{#Babel:de|fr|ar}}
• disable js
• go to https://wikidata.beta.wmflabs.org/wiki/Item:Q11?useformat=mobile
• see the selected languages + the user interface language
• 🎉️