Default Searchbox does not produce results for non-English headers

[WahaWaher]

• I confirm that this is an issue rather than a question.

Bug report

Hi! I use the default VuePress search. My project has headers in different languages. Search requests in English work fine, but for Russian headers - the result is empty. One of my pages contains the following:

# English title
...
## English Sub title
...
### Заголовок на русском языке
...

When you enter "heading" in the search field - no results. Is this a bug or I doing something wrong? Thanks in advance for reply.

What is expected?

After entering "заголовок" in Russian into SearchBox, I expect to receive a search result with the heading "Заголовок на русском языке"

What is actually happening?

For queries in English I get a result, for queries in Russian I get nothing

Other relevant information

• Output of npx vuepress info in my VuePress project:
  System:
  OS: Windows 10 10.0.18363
  CPU: (4) x64 Intel(R) Core(TM) i3-3110M CPU @ 2.40GHz
  Binaries:
  Node: 12.16.1 - D:\dev\programs\nodejs\node.EXE
  Yarn: Not Found
  npm: 6.13.4 - D:\dev\programs\nodejs\npm.CMD
  Browsers:
  Edge: 44.18362.449.0
  npmPackages:
  @vuepress/core: Not Found
  @vuepress/theme-default: Not Found
  vuepress: 1.4.0
  npmGlobalPackages:
  vuepress: Not Found

[psalaets]

Possible duplicate of #2242?