TYPO3 Core

Correction: it seems string keys are not possible there.

So the MRE is:

config.concatenateJs = 1  
page.jsInline {
  10 = TEXT
  10.value = alert(1)
  20 = TEXT
  20.value = alert(2)
}

My debugging went as far as to understand that page.jsInline is quite different from the others like page.includeJS.

page.jsInline renders no separators because it gets handed to \TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer::cObjGet directly.

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/+/77320

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/+/77320

Patch set 4 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/+/77320

Patch set 5 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/+/77320

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/+/81686

Patch set 1 for branch 12.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/81687

Patch set 2 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/+/81686

Please reopen, because applied patch makes unnecessary BC at least on 11.5.
See:

    public function cObjGet($setup, $addKey = '')
    {
        return implode('', $this->cObjGetSeparated($setup, $addKey));
    }

    public function cObjGetSeparated(?array $setup, string $addKey = ''): array
    {
        if (!is_array($setup) || $setup === []) {
            return [];
        }

Proposal:

    public function cObjGet($setup, $addKey = '')
    {
        if (!is_array($setup)) {
            return '';
        }
        return implode('', $this->cObjGetSeparated($setup, $addKey));
    }

    public function cObjGetSeparated(?array $setup = [], string $addKey = ''): array
    {
        if (empty($setup)) {
            return [];
        }

Do you mean it became stricter? I agree, and I am sorry I have not noticed that. Before, it would return '' when given anything but an array.

Sorry, what is "BC"?

Thanks for the report Rafael.
Please find a fix in:

81698: [BUGFIX] Fix possible regression in cObjGet() | https://review.typo3.org/c/Packages/TYPO3.CMS/+/81698