SSIS Data Conversion Redirecting Rows to Error Output Despite Valid Data

I have a Data Conversion component in SSIS that keeps redirecting rows to the error output despite the data being valid. The FIRE_SIZE_CLASS column only contains single characters A-G, confirmed through inspection, and the conversion is set to DT_STR with a length of 1 and code page 1250. However, thousands of rows are still sent to the error output with the error:

Data conversion failed while converting column "FIRE_SIZE_CLASS" (359) to column "FIRE_SIZE_CLASS" (141). The conversion returned status value 4 and status text "Text was truncated or one or more characters had no match in the target code page."

The Data Viewer shows only valid single characters, no extra spaces, nulls, or unexpected characters. What could be causing this truncation error in SSIS?