TYPO3 Core

Some more debugging information:

while static select icons have a syntax like 'EXT:styleguide/Resources/Public/Icons/tx_styleguide.svg', icons of releations using ctrl|selicon_field have a path like /fileadmin/styleguide/telephone_box.jpg which is created in

AbstractItemProvider

if ($isReferenceField) {
                    $references = $fileRepository->findByRelation($foreignTable, $iconFieldName, $foreignRow['uid']);
                    if (is_array($references) && !empty($references)) {
                        $icon = reset($references);
                        $icon = $icon->getPublicUrl();
                    }
                }

later in \TYPO3\CMS\Backend\Form\Utility\FormEngineUtility::getIconHtml the line

$absoluteFilePath = GeneralUtility::getFileAbsFileName($icon);

will transform the fileadmin path into an empty string and the icon can be retrieved

Found causing commit through git bisect: #95027

The difference is the prefixed slash in /fileadmin/styleguide/telephone_box.jpg
Without the slash it works again: fileadmin/styleguide/telephone_box.jpg

Patch set 2 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/77208

Patch set 3 for branch main of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/77208

Patch set 1 for branch 11.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/77431