The issue has to do with the OLEDB provider which LinqToExcel uses.
Here's a stack overflow question that explains the issue: Excel cell-values are truncated by OLEDB-provider
The article explains that adding HDR=YES helps with the problem, and LinqToExcel already sets that property.
So basically you need to have an alphanumeric room number in the first 8 or 16 rows. Else OLEDB treats all the values in that column as numbers instead of strings.