SELECT returns null for string values

I have a measurement that contains some string values. I’m in the process of converting the string values to floats, and I first need to extract the current data.

SELECT PM2_5Value FROM “purpleair” WHERE time > ‘2020-08-01’

Returns the timestamp and string values properly.

If I run the following though, those string values now return null.

SELECT * FROM “purpleair” WHERE time > ‘2020-01-01’

This doesn’t make any sense to me, any help would be appreciated!