Thank you for your post.
A repeating field is a single field that contains many values. Each repetition is separated by a control character (ASCII 29). If the first instance is displaying the same information for each record, but the other repetitions are not affected, then there may be some corruption in the field.
The next time this occurs, try closing the file, quit and relaunch FileMaker Pro, and open the file. If the correct data appears, then this may be a memory issue. If the incorrect data still appears, then close the file, and run a Recover. When the recover finishes, open the recovered file. This may or may not show the correct data. If it doesn't show the correct data, then import the repeating data from the last good backup file, and use the option to update the existing records.
Let me know if you continue to run into difficulty.
In FileMaker, it's relatively easy to use Import Records to move data from a repeating field or group of repeating fields into a set of related tables with one related record for each value in a repetition.
This may be a good time to consider making that design change for this data.