Google commissioned Jacques Le Bailly @Fonthausen to do an extensive revision of the latin character set.
We applied a new weight distribution across the variables.
Now Montserrat has extended Cyrillic support (GF Cyrillic Pro).
This is a major update. The new version is better but will make the text reflow in most cases.
The old regular (400) –version 6.002– is closer to the new Medium (500). If you want to maintain the appearance of the text you can try to switch to Medium, or you can use the old version, which is available for download (or use as a self-hosted webfont) in https://github.com/JulietaUla/Montserrat/releases/tag/v6.002