REST API: Fix embedded items 'context' regression #2732

Job	Run time
Core running from build / Core running from build / Linux	1m 56s
Core running from src / Core running from src / Linux	1m 54s
Core running from build / Core running from build / Windows	3m 11s
Core running from src / Core running from src / Windows	3m 40s
Core running from build / Core running from build / MacOS	7m 10s
Core running from src / Core running from src / MacOS	3m 44s
Gutenberg running from src / Gutenberg running from src / Linux	8m 5s
Gutenberg running from build / Gutenberg running from build / Linux	8m 17s
Gutenberg running from build / Gutenberg running from build / Windows	12m 1s
Gutenberg running from src / Gutenberg running from src / Windows	12m 16s
Gutenberg running from src / Gutenberg running from src / MacOS	10m 18s
Gutenberg running from build / Gutenberg running from build / MacOS	15m 43s
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	1h 28m 20s

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