Yes, it is relatively common for IPUMS USA variables that are continuous to also include values that are categorical. You can view the labels for these values using ipums_val_labels() or by viewing the INCTOT variable (e.g. if your data was named data you could do data$INCTOT). Information from both the catgry and codInstr tags are captured here.
These codes are also mentioned in the codes tab for each variable. The codes tab for INCTOT notes these as specific variable codes. This is most commonly used for missing and NIU data, but it is also used in other cases such as for bottom and top codes. For example, all respondents who report an INCTOT value above a certain threshold are assigned the same top code by the Census Bureau to preserve confidentiality (see the IPUMS User Guide page on threshold values). This is also mentioned in the codes tab on the website. We recommend that users review the codes tabs for all variables in their extract in order to code respondents correctly for their analysis.