On what basis DMA identifies columns for Dynamic Data Masking?
From what I understand, DMA looks at the column names and tables for keywords that would hold PII, Sensitive Data (such as logins, passwords, etc.), or other unique data attributes. If it finds anything then it should mark it for both dynamic data masking and always encrypted.

If you change the name of the column to not have easily identifiable keywords, the recommendation should not show up for that column/table.