We need a little more detail to look into this. The search handles text and choice fields differently, so knowing the exact field type, the characters involved and how the field was created will show which path is failing.
Which custom field type fails: single-line text, multi-line text, drop-down/radio, or checkboxes/multi-select?
Can you give an example of a stored value (or choice ID and label) and the exact search term that fails to return the user?
Was the field created in the admin panel, or did it come from an importer or a direct database change? If it's a choice field, do any choice IDs contain non-English characters?
Does the failing text include emoji or other 4-byte characters, and is full Unicode (utf8mb4) enabled on the affected site?
This is an automated assessment. A member of staff may review and update it.