EVA construction mode while on the surface lets left-shift "bleed through" to thrust the jetpack when placing or adjusting a part.
When an EVA engineer is in construction mode, you are supposed to have full access to the construction widgets and presumably that includes holding left-shift down in order to make angle-snap use smaller increments. (i.e. angle-snap without left-shift rounds to 15 degrees, but with left-shift it rounds to only 5.)
But if you entered construction mode while RCS was on, that left-shift key still causes your kerbal jetpack to thrust upward, defeating the purpose since you can't hold the camera still to adjust the part because your Kerbal is accelerating away.
What it does: In EVA construction mode while a part is being placed, moved, or rotated, left-shift activates BOTH fine-grain angle-snap and thrusts the jetpack upward.
What it should do: In EVA construction mode while a part is being placed, moved, or rotated, left-shift only activates fine-grain angle-snap and does NOT also thrust the jetpack. Once the part is no longer in hand, then it's okay to make the jetpack controls start working again.