Matrix blocks are no longer propagating across site entries on first save

[mmikkel]

Description

Since Craft 3.0.27, Matrix fields that are set to manage blocks on a per-site basis would duplicate Matrix blocks across all of the owner element's supported sites, when that element was first created (see relevant discussion in #3082)

In Craft 3.1.32.1, this is no longer the case – no blocks are duplicated across site entries, for a non-propagating Matrix field in a propagating Section.

I tested this on a clean Craft 3.1.32.1 install, without any plugins installed.

I also did a bunch of testing on earlier 3.1.x builds, and it appears that this issue first occurs in 3.1.28 – everything works as expected, prior to that release.

I can't see any notes in the changelog about this being intentionally reverted back to its <3.0.27 behaviour, so I'm assuming this is a bug?

Steps to reproduce

1. Create a Section that is propagating across multiple sites
2. Add a Matrix field to that Section, that has the "Manage blocks on a per-site basis" checkbox checked
3. Create a new Entry, and add some blocks to the Matrix field before saving the Entry
4. Confirm that no blocks were duplicated to the other site versions of the Entry

Additional info

• Craft version: 3.1.28, 3.1.32.1 (works as expected prior to 3.1.28)
• PHP version: 7.2.14
• Database driver & version: MySQL 5.6.42
• Plugins & versions: none

[brandonkelly]

Looks like 0a88191 is the culprit.

The fix is a little complicated, and all surrounding code has been refactored in Craft 3.2. Since 3.2 is so close to a stable release, I’ve fixed this there (for 3.2.0-RC2) rather than for 3.1.

To get the fix early, change your craftcms/cms requirement in composer.json to:

"require": {
  "craftcms/cms": "3.2.x-dev#561353619b03f16dd8b3694141d1c371467577b4 as 3.2.0-RC1",
  "...": "..."
}

Then run composer update.