=== Repro Steps
Run this command a number of times and observe the title of the page in the output. Using only the title here for simplicity, the whole page has the same problem.
```curl -sv -H "Accept-Language: zh-hant" -H "Cache-Control: no-cache" https://zh.wikipedia.org/api/rest_v1/page/mobile-html/2019%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85 | egrep -ohi '(?:<h1 data-id="0" class="pcs-edit-section-title">)[^<]+'```
=== Expected results
The language is zh-hant every time
=== Actual results
Sometimes the language is zh-hans
Here's an example request with the correct variant (only including the relevant headers):
```
> GET /api/rest_v1/page/mobile-html/2019冠状病毒病疫情%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85 HTTP/2
> Host: zh.wikipedia.org
> User-Agent: curl/7.64.1
> Accept: */*
> Accept-Language: zh-hant
> Cache-Control: no-cache
...
< content-language: zh-hanttype: text/html; charset=utf-8; profile="https://www.mediawiki.org/wiki/Specs/Mobile-HTML/1.2.1"
< vary: Accept-Language, Accept-Encoding
< content-language: zh-hant
< date: FriTue, 0307 Apr 2020 09:57:3311:48:05 GMT
< cache-control: s-maxage=1209600, max-age=0, must-revalidate
< x-request-id: b1f4750b-ab56-451a-8e18-2ac07882bf19access-control-allow-methods: GET,HEAD
< referrer-policy: origin-when-cross-origin
< server: restbase1026content-location: https://zh.wikipedia.org/api/rest_v1/page/mobile-html/2019%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85
< etag: W/"58953070/8a350660-7591-11ea-8066-dfe689212e6d"x-request-id: 73184458-76e5-47bc-bb1d-38167a6a2a30
< age: 1server: restbase1027
< etag: W/"59036991/a46ba570-78c5-11ea-8284-a649b8c658ff"
< age: 4
< x-cache: cp107581 miss, cp10837 pass
< x-cache-status: pass
< server-timing: cache;desc="pass"
< strict-transport-security: max-age=106384710; includeSubDomains; preload
```
And one with the incorrect variant:
```
> GET /api/rest_v1/page/mobile-html/2019冠状病毒病疫情%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85 HTTP/2
> Host: zh.wikipedia.org
> User-Agent: curl/7.64.1
> Accept: */*
> Accept-Language: zh-hant
> Cache-Control: no-cache
...
< content-language: zhtype: text/html; charset=utf-8; profile="https://www.mediawiki.org/wiki/Specs/Mobile-HTML/1.2.1"
< vary: Accept-Language, Accept-Encoding
< content-language: zh
< date: FriTue, 0307 Apr 2020 09:48:4711:45:34 GMT
< cache-control: s-maxage=1209600, max-age=0, must-revalidate
< x-request-id: 0f208c8f-f961-4e63-88ad-0e102d082f36access-control-allow-methods: GET,HEAD
< referrer-policy: origin-when-cross-origin
< content-location: https://zh.wikipedia.org/api/rest_v1/page/mobile-html/2019%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85
< server: restbase1019x-request-id: 889b90a3-29de-4f8b-9589-296275b69a99
< etag: W/"58953070/503dcfb0-7590-11ea-9def-6cf346db56c5"server: restbase1020
< age: 529etag: W/"59036991/4af063a0-78c5-11ea-a66a-a2018dda4efe"
< x-cache: cp1083 hit, cp1083 passage: 7
< x-cache: cp1083 miss, cp1087 pass
< x-cache-status: hit-locals: pass
< server-timing: cache;desc="pass"
< server-timing: cache;desc="hit-local"strict-transport-security: max-age=106384710; includeSubDomains; preload
```
When running it against the mobileapps service labs instance, I see the correct title every time:
```curl -sv -H "Accept-Language: zh-hant" -H "Cache-Control: no-cache" https://mobileapps.wmflabs.org/zh.wikipedia.org/v1/page/mobile-html/2019%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85 | egrep -ohi '(?:<h1 data-id="0" class="pcs-edit-section-title">)[^<]+'```
=== Additional info
I'm seeing the same problem on mobile-sections-lead, so it appears the issue is not limited to mobile-html:
```curl -sv -H "Accept-Language: zh-hant" -H "Cache-Control: no-cache" https://zh.wikipedia.org/api/rest_v1/page/mobile-sections-lead/2019%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85 | egrep -ohi '"displaytitle":"[^"]+"'```
== Original bug report:
This can be reproduced on the Wikipedia Android beta app.
Please note that this issue does not happen on every `zh.wiki` articles.
=== Steps to reproduce ===
1. Open article: [[ https://zh.wikipedia.org/wiki/2019%E5%86%A0%E7%8A%B6%E7%97%85%E6%AF%92%E7%97%85%E7%96%AB%E6%83%85 | 嚴重特殊傳染性肺炎疫情]] in Traditional Chinese
2. Refresh the page after it's fully loaded.
3. You will see the article's variant is changed which includes the article content, ToC data, and footer.
DEMO: https://www.youtube.com/watch?v=_gZ8lMg5O60