Believe I found a bug in Photoshop. Hope someone else can perform this test.
I believe if you have a document with a layer that is a smart object using an input ICC profile inside the smart object with a whitepoint lower than L* 100, and said document is still using that input ICC profile, and you use convert to profile outside of the smart object, the rendering intent you tell Photoshop is ignored and replaced instead with the default rendering intent in color settings.
A little while ago, I decided to implement a non-destructive scanner ICC profile to working space conversion. Meaning, keeping the original RGB values from the scan tucked away in the Photoshop file. I would have a smart object, that inside was left as the scanner's ICC profile. Outside the smart object is where the conversion to ProPhoto RGB would take place. If at a later time I changed to a new scanner ICC profile, I could go in that smart object and assign the new one. If I had made color adjustments after that process, those of course would have to be re-evaluated. However, any touchup work like hair removal wouldn't need to be redone.
Here's the steps to try reproducing:
(1) Create a new document. Fill it with pure white - RGB 255/255/255. Assign your scanner's input profile. This simulates if your scanner created a document filled with the brightest white it can see. At this point, if you use eyedropper or Info panel, you will see either L* 100 if your color settings intent is Relative Colorimetric, or L* of your scanner's profile's whitepoint if your color settings intent is Absolute Colorimetric.
(2) Select all layers, and right click selecting Convert to Smart Object.
(3) If your document is in 8bit mode, select Image->Mode->16Bits/Channel.
(4) Save document as "Common.PSD".
(5) Edit->Color Settings. Under Conversion Options, make sure intent is Absolute Colorimetric. (If you don't see Conversion Options, click the "More Options" button on the right.)
(6) Edit->Convert to Profile. Select ProPhoto RGB, Intent Absolute Colorimetric.
(7) Flatten document
(8 ) Save document as "ColorSettingsAbsolute.tif"
(9) Re-open "Common.PSD".
(10) Edit->Color Settings. Under Conversion Options, change intent to Relative Colorimetric.
(11) Edit->Convert to Profile. Select ProPhoto RGB, Intent Absolute Colorimetric. ***NOTE again absolute here - This is the WHOLE point of the test here - NOT relative like you set in Color Settings***
(12) Flatten document
(13) Save document as "ColorSettingsRelative.tif"
(14) Close document, and open "ColorSettingsAbsolute.tif" and "ColorSettingsRelative.tif".
The files should be identical. You told Photoshop in both cases to convert using absolute, so the white point of your scanner would be left alone and not mapped to L* 100. On my CS6 (v13) and CC (v14.1.1), these files are instead different. "ColorSettingsRelative" is brighter in appearance, RGB, and LAB values.