In brief
A file for Regular is not automatically a real Bold.
Useful to know
@font-face describes a font resource and the features with which it should be used. Family name, source, style, and severity link the file to subsequent CSS queries. [1]
Example: @font-face { font-family: "Example"; src: url("example.woff2") format("woff2"); font-weight: 400; font-style: normal; }. For a separate Bold file, a corresponding description is required. In variable font, an appropriate range is set according to the resource. Check if the file itself contains the required Cyrillic characters and if you are eligible for web use; the declaration does not add missing glyphs or a license.
Sources
For further detail and to check the information: