Steps to reproduce the issue

In the Joomla! backend go to "Extensions > Language(s) > Overrides", and click on "New" to create a new override. Into the field labeled "Language Constant" enter

TEST_CONSTANT

Into the field labeled "Text" enter the following HTML markup code which includes a valid attribute:

<span data-type="abc">A</span>

Click on "Save".

Expected result

<span data-type="abc">A</span>

Actual result

<span datatype="abc">A</span>

Mind: datatype as opposed to the originally entered data-type with a hyphen.

System information (as much as possible)

Joomla! 3.8.8

Additional comments

The hyphen is silently stripped from the attribute name. As far as I can see this is wrong and should be fixed.

I think it is important to mention that the hyphen in a class name — or elsewhere in the text — is not stripped. The following is left untouched when the override is saved:

<span class="my-class">A</span>

Similar for plain text:

Swimming-Pool

Whatever "corrects" the value does not just strip hyphens but seems to make a distinction between HTML attribute names and other strings.

According to the W3C documentation a hyphen in an HTML attribute name is valid:
https://www.w3.org/TR/2011/WD-html5-20110525/elements.html#embedding-custom-non-visible-data-with-the-data-attributes
So a "syntax correction" — if that is what happens here — would not be appropriate.

(As a workaround the text can be entered manually into the *.ini file, of course.)