searching titles and content is something that is part of WP and not something that is specific to ACF. Basically, ACF does a standard query with the ‘s’ parameter set. This can be altered with quite a bit of work, but I don’t know if you can limit it only to the query being done by the relationship field.
I found some code here for removing the content from search. https://nathaningram.com/restricting-wordpress-search-to-titles-only/
The problem is that I do not think there is anything that will identify the query as being for the relationship field.
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