There are actually two issues here:

1. OOUI styling for focussed multiline text inputs in invalid state is wrong. I think it has been wrong for more than a year, since d430b0c90, which made a minor tweak to the focus styles but forgot to style the invalid state. You can reproduce the same incorrect styling on en.wp by opening this dialog, typing into the field, then deleting everything.
2. VisualEditor marks the field as invalid when the dialog initially opens. OOUI has a bunch of messy code aiming to only mark focussed fields as invalid after the user interacts with them (even if their initial value is invalid, like here). We are now doing something that breaks it. I don't know yet why this happens, but it might be another issue revealed by T185944.

Change 446707 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[oojs/ui@master] WikimediaUI theme: Fix styling for focussed multiline text inputs in invalid state

https://gerrit.wikimedia.org/r/446707

Change 446708 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[mediawiki/extensions/VisualEditor@master] ve.ui.MWTemplateDialog: Prevent focus changes in the setup process

https://gerrit.wikimedia.org/r/446708

@Ryasmeen Although I've already put this on resolved, please have a look after next OOUI release (next or second-next week).

Change 446707 merged by jenkins-bot:
[oojs/ui@master] WikimediaUI theme: Fix styling for focussed multiline text inputs in invalid state

https://gerrit.wikimedia.org/r/446707

Change 446708 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] ve.ui.MWTemplateDialog: Prevent focus changes in the setup process

https://gerrit.wikimedia.org/r/446708

Deployment / QA note:

• The VisualEditor fix is now live in Beta and will go to production next week (unless backported).
• The OOUI fix will probably be included in next week's release, which is when it will go live in Beta, and will go to production the week after that (unless backported).