Final Cut draw mask drops when editing & exporting

I can recreate this easily enough:

When I did this, I have the Draw Mask on one "layer" and a "transforming effect" added afterwards which changes the position and scale of the media. The Draw Mask shows when the effect is selected in the Inspector in its original position drawn.

Do you have any other effects added *after* the Draw Mask?

Effects are "layer ordered" in Final Cut in "Top Down" order. Each effect added affects all that went before it.

Layer order can be changed easily by clicking and dragging the effect's "titlebar" to a new position in the order in the inspector.

Has the Draw Mask worked OK for you previously?

Try this:-

As you launch FCP X hold down ⌘ and ⌥ until a window pops up.

Select Delete Preferences and when FCP X opens navigate to the Library you were working on and open it.

Shut down FCP.

Hold down ⌘ and ⌥ and keep holding them before you launch FCP.

Launch FCP and don't let go of those keys until the Delete Preferences window appears.

Then click the Delete button.

How many clips are there in the timeline at this location? Which clip are you applying the draw mask to?

Can you show us the timeline at this location?

Regarding the Command and Option, as Ian suggested (and I second): press and hold Command and Option immediately after launching FCP - for example, if you launch using Spotlight, press enter and immediately press and hold the two keys; only release when the dialog appears.

Can you post some screenshots showing the draw mask and the results of your export?