Issue with \definetextbackground in bidirectional text (English + Arabic)

Here's an example of mixed English and Arabic text. I defined the
`HighlightText` command to highlight the text, but a problem arises in both
cases:

   1.  In  case of right-to-left text, the command fails with English text
   (If it is not followed by English text).
   2.  In  case of left-to-right text, the command fails with Arabic text
   (If it is not followed by Arabic text).

```
\starttext
\setupdirections[bidi=global,method=one]

\definefont[myfont][name:Amiri*arabic at 12pt]

\definetextbackground[HighlightText][backgroundcolor=yellow, frame=off]

\startmixedcolumns[n=2]
\myfont

\righttoleft

first word
\startHighlightText
second word
\stopHighlightText

كلمة أولى
\startHighlightText
كلمة ثانية
\stopHighlightText

\column

\lefttoright

first word
\startHighlightText
second word
\stopHighlightText

كلمة أولى
\startHighlightText
كلمة ثانية
\stopHighlightText

\stopmixedcolumns

\stoptext
```

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