Environment

Adobe Acrobat (unified app) 26.002.22031, also reproduced on 26.002.21901
Running in Reader mode (not signed in). Crash reporter tags report CR_AdobeProductName = AcrobatReader
macOS 26.3.1 (25D2128), Apple Silicon (arm64)

Steps to reproduce

Fully uninstall Acrobat and remove all user and system data: ~/Library/Application Support/Adobe/Acrobat, ~/Library/Containers/com.adobe.Acrobat.Pro, ~/Library/Preferences/com.adobe.Acrobat.Pro.plist, caches and ARMDC data.
Install Acrobat 26.002.22031 and launch it without signing in.
Preferences → Trust Manager → Load trusted certificates from an Adobe EUTL server is enabled. Click Update Now.
Open a PDF signed with an EU qualified certificate whose root CA is on the EUTL.

Expected result
The EUTL trust anchors are added to the trusted identities, and the signature validates as a qualified signature.

Actual result

The signature shows “Signature validity is unknown – Signer’s identity is unknown because it has not been included in your list of trusted certificates and none of its parent certificates are trusted certificates”. The embedded timestamp cannot be verified either.
Settings → Trusted Certificates lists only Adobe Root CA and Adobe Root CA G2.
Clicking Update Now shows no dialog, no confirmation and no error.

Evidence that the download works but the import never happens

The Synchronizer DB (~/Library/Application Support/Adobe/Acrobat/DC/Acrobat/Synchronizer/metadata/Synchronizer) records https://trustlist.adobe.com/eutl12.acrobatsecuritysettings with HTTP 200, state 2 and a sync timestamp. The cached resources/resource-N file (4,550,731 bytes) is byte-identical (SHA-256) to the file on the server, dated 2026-10-02. Its signature by Adobe Certified Document Service Program is valid.
Preferences DC/Security/DigSig/EUTLDownload are updated: LastChecked set, ResourceID set, WasFallback = false.
~/Library/Application Support/Adobe/Acrobat/DC/Security/addressbook.acrodata is never modified after the download. It keeps the 5 default entries and 14,451 bytes, also after quitting Acrobat.
Setting AskBeforeInstalling = false for EUTLDownload makes no difference.
Opening eutl12.acrobatsecuritysettings directly in Acrobat shows the certified PDF with the SecuritySettings.xml attachment, but no import prompt.
An older addressbook.acrodata_old from May 2026 on the same machine contained about 1,800 entries with Source [(EUTL)]. The feature used to work, so this looks like a regression.

Workaround
Writing the EUTL identities from SecuritySettings.xml into addressbook.acrodata (Trust 96/98, Source [(EUTL)]) with Acrobat closed makes the signature validate correctly. This confirms that the trust list content and the signature are fine, and that only the import step fails.