There are only 2 Tweets in your timeline eligible to be returned on a search call (within the past 7 days). It does look like one which has a
source of Instagram is not showing up, but there are several reasons why that may be - the standard search API index is not necessarily complete. The same Tweet does show up in the web search, which uses a different indexing and relevance mechanism. I’m not aware of a specific limitation here, and I can find Tweets from other users posted via the Instagram app on the API search index, so I suspect this is just an anomaly.