Missingness in blood pressure medication data

Hi, I have a question regarding blood pressure medication variables (captured in field IDs 6177 and 6153)

In my analytical sample, these variables have a substantial amount of missing data, with over 50% of participants coded as NA. It also appears that the answers provided to these variables/questions were reviewed again during the nurse interview, which makes the extent of missingness in fields 6177 and 6153 difficult to understand.

I'd appreciate any guidance on the following: 

1) Are there any known reasons why fields 6177 and 6153 contain missingness? 

2) How is the missingness in these types of variables usually interpreted? - Should NAs be treated as genuine missing data, or can it be assumed that participants who are NA were not taking antihypertensive medication (or any of the other medication categories captured by these fields)?

3) Are there alternative fields that could be used, that may provide more complete information on blood pressure medication use?

Any insight on what might be driving this missingness in these variables would be really helpful

Thank you