If you have subfields or repeater fields in ACF, Relevanssi doesn’t understand the
field_%_subfield notation. You can have Relevanssi index subfields by setting the custom field setting to
all: that makes Relevanssi index all custom fields, which includes the repeater fields.
If you want more control, Simon Wheatley has created Relevanssi: add ACF subfields to index plugin that finds subfields from ACF and feeds them to the Relevanssi indexer so they’re findable in search. It’s simple and effective. With this plugin enabled, you can use
field_%_subfield to add subfields to Relevanssi index.
Update: the plugin is somewhat outdated, and this functionality has been part of Relevanssi Premium from version 1.15.3.