I don't know if this worked in AC18/19, but it definitely works now: with the Label tool active, hover you cursor over the Door Panel you want to label, then press the TAB key to cycle through available items to Label (whole Curtain Wall, Door Panel), then click to label when the Door Panel is highlighted with the pre-selection highlight. The Info Tag will also tell you what you are about to label.
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2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac27