Site Title Block changes font size when viewed as a template part

[annezazu]

Describe the bug

The Site Title Block changes when viewed as a header template part vs as part of the wider front page template.

To reproduce
Steps to reproduce the behavior:

1. Head to Gutenberg > Experiments and check both "Enable Full Site Editing" + "Enable Full Site Editing demo templates".
2. Edit the Site Title Block.
3. Use the Template Selector to select Template Part > Header.
4. Notice the difference in size of the font of the Site Title Block.

Expected behavior

I expected the Site Title Block font size to remain unchanged.

Screenshots

Here's a video: https://cloudup.com/c0IAFN0MVBf

View of header template part:

View as part of overall front page template:

Editor version (please complete the following information):

• WordPress version: 5.4.1
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? Gutenberg Plugin
• If the Gutenberg plugin is installed, which version is it? 8.1.0

Desktop (please complete the following information):

• OS: MacOS
• Browser: Chrome
• Version: 81.0.4044.138

[alaczek]

Could this possibly be theme related? I tested in Seedlet blocks theme and the font-size remains the same:

View of header template part:

[carolinan]

Twenty Twenty Blocks is not maintained so I was not able to test with that theme.
I am closing this issue as I am not able to reproduce it in TT1 Blocks, empty theme, Q or Armando.