Enter layout mode, select the field. Select the Data tab in the Inspector. Scroll to the bottom of this tab.
What data format settings have been specified for this field?
Enter browse mode.
Click the field to put the cursor in it. Does the appearance of the data change?
If so, what do you see?
Do both layouts refer to the same Tutorial: What are Table Occurrences??
There is a possibility of file corruption, index corruption, or other problems.
A database that has been converted once or more is a likely candicate for these problems.
Backups are good, recovery can help diagnose such symptoms.