In the pop-up for "Find by caption or note", the are - in addition to the string to be searched for - two conditions: (1) "Match only the beginning..." and (2) "Match any part...". Condition 1 is more restrictive than condition 2 and therefore will yield an equal or smaller number of search results. Condition 2 is more permissive than condition 1 and therefore will yield an equal or larger number of search results. Condition 1 is the default condition. I would prefer condition 2 to be the default, because I unvariably find myself to override the default and select condition 2. For the vast majority of my caption searches, I don't know the exact caption; instead, I vaguely remember or suspect a word that might be a part of the caption.
That's a long time I have stopped to use the old 'detail' search when I have to search by any substring of a caption or note.
Did you try the smart search function ? (the magnifying glass 'search' on the top right corner?)
I just tried a part of the caption 'de Vinc' for the caption or keyword 'Bois de Vincennes' and got nearly instantly 755 items. I added 'Arth' to find my grandson Arthur and I got 85 items in my library of 80.000 images.