ifelse test returns NA when data value is within specified parameters

I'm attempting to apply size classes to trees based on their diameter at breast height (dbh) measurement.

I have the following code to classify them, repeated 19 times.

    data_dbh$size_class_12 = ifelse(data_dbh$dbh1 %in% c(5.0:9.9), "A",
                            ifelse(data_dbh$dbh1 %in% c(10.0:19.9), "B",
                                   ifelse(data_dbh$dbh1 %in% c(20.0:39.9), "C",
                                          ifelse(data_dbh$dbh1 > 39.9, "D", NA))))

Some of the newly created columns in the dataset have NA, when the value in the respective dbh column is within the specified %in% range, such as 14.9, which ought to have returned B; this also occurs in the other %in% ranges.

I've tried str_trim with side = "both" to clear possible spaces, as well as as.numeric and as.character, but nothing changes.

I'm fairly new to R and bad in understanding programming language in general, so if any answers could be simplified as far as possible, that would be great.