Description

When I switch the active block theme, wide and full alignments are lost on the page using a custom template.

Maybe related to #48074 and #47477, but this also happens with default block themes such as TT2 and TT3.

Tested with WordPress 6.2 beta3 nightly.
Same result with/without Gutenberg plugin 15.2.

Step-by-step reproduction instructions

1. Use TT2 theme, create a new page and set its template to "Page (Large Header)"
2. Add blocks and set its alignment to wide or full.
3. Switch active theme to TT3. Then, the template of this page is automatically set to "Default" and wide/full alignments are lost.

Screenshots, screen recording, code snippet

Before switching theme (TT2)	Template

After switching theme (TT3)	Template

However, wide/full alignments are back if I change the template after switching theme:

Change template after switching theme (TT3)	Template

Environment info

WordPress 6.2-beta3-55409 (latest nightly), instawp install
TT2 and TT3 included in nightly
Tested with and without Gutenberg plugin 15.2

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes