WP 4.9 doesn't recognize changes/additions to existing style sheet, forcing use of "Additional CSS" feature as sole css editor

[Riavon]

WP 4.9 is preventing changes/additions made to an existing external style sheet from being reflected on the front end of the site.

Additional styles added to existing theme/child theme external style sheets after upgrading to WP 4.9 are no longer recognized. I was forced to use the “Additional CSS” feature in the Dashboard in order to change/add new css styles to show up on the front end of the site. The front end of the site still reflects pre 4.9 upgrade styles from the external style.css, but after 4.9 upgrade any new/added styles will not show on the front end of the site. It's forcing me now to use the built in "Additional CSS" feature as the only way to see any new/added/changed styles on the front end.

WP website development, started with WP 4.83, was upgraded to WP 4.9 during development and this bug happened.

I don’t use the theme editor in the Dashboard to write my CSS. I'm a pro WP designer with 10+ yrs of WP experience, so I know that clearing the cache is the first line of defense, and yes I ran all the tests: tested in different themes including the default theme, all the browsers, etc.,etc.

I prefer to continue development of this and all of my WP sites using the external style sheet editor I started with before I did the upgrade to 4.9, but with 4.9 it’s not possible.

NOTE: When I rolled back to WP 4.83 and re-tested everything, all is well. Everything with the style sheet/css is back to working perfectly; i.e additions and changes made to the existing style sheet are once again showing on the front end with WP 4.83.

[Riavon]

Replying to Riavon:
This appears to be a theme compatibility issue. Will take it up with the theme developer.

[kraftbj]

Thanks for following up and closing the report once the theme issue was discovered! We appreciate the report.