I cannot confirm your find with Joomla 3.9.19-dev (current staging).

That's the configuration I've had. Used to work normally. Suddenly, the Browser page title got replaced by the Page Heading. Nothing else had changed, except Joomla's recent update.

Yes, I understand but cannot reproduce it. I've also tested with my own site (3.9.18) where I have nearly in any menu item different Browser Page Title and Page Heading entries.

I tested with menu item types:
Articles » Featured Articles
Articles » Single Article
Articles » List All Categories
Articles » Category Blog

Firefox, Chrome

@MaghSamana I wonder if your case has something to do with some special characters. Maybe the page heading is a fallback when the browser page title cannot be read/processed correctly.

Found the issue: it was a cache problem from a disabled component (EFFSEO). I still don't know why with the Joomla update the cache of the disabled component was mixed in, but once cleared the problem stopped.

Gerryfrancis: I've been using special characters a lot, so I can say for sure that Joomla system handles them fine.

@MaghSamana Great that you have found the solution, and thanks for letting us know. Please close here, thank you.

Closing as issue is solved in 3rd party component.