Sorry,
but unfortunately “auto mask” is broken by design. Right after it was introduced with PL 1.0, I explained its problems and suggested a solution/fix - which was promised shortly after, but never has been implemented. That’s four years ago now…
You might get some impressions here:
Local Adjustments in new Photolab 1.2
Artifacts using clearview plus on local adjustments - #16 by Tilmann
Local adjustment: Automatic mask tool: display of effective mask
Still, auto mask is the only local adjustment for which you can’t see or correct what finally gets picked, it’s complete usage is inconsistent to all other local adjustments.
The mentioned workaround with extreme exposure settings is slow and inefficient, however at the time being, it’s the only way to at least get some impression what’s picked.
Since DxO never reacted at all (regarding this and some other long-standing issues) for years now, one might fear there will never be a fix. 