Maybe this change history entry from the POV beta page might give you helpful
clues:

---snip---

Alpha Changes
  -------------

  Some changes have been made to the way alpha is handled when +UA is activated.
  Firstly, in previous versions, specifying a background with the background
  keyword would by default supply a background with transmit set to 1.0 (i.e.
  fully transparent provided that +ua is being used). This is no longer the
case.

  While the default background is transparent, any background specified in a
  scene file (unless 3.6 or earlier compatibility is being used) will now be
  opaque unless transmit is explicitly given. (In other words, use rgbft<>
  rather than rgb<> in the background statement if you want the old behaviour).

  Secondly, the way that objects are blended with the background has changed.
  Previously the color of the background was not taken into account when
  calculating effects of transmission through translucent objects when +ua is
  in effect (i.e. where the background could otherwise have been seen through
  the object). Now, however, the background color is taken into account, even
  if it is not otherwise visible. (In other words, blending is performed in the
  same way regardless of the presence of background transparency).

  Note that this change is not affected by the #version directive, so it may
  change the appearance of scenes written for version 3.6 and earlier. We will
  monitor feedback on this from beta testers to evaluate the impact of this.

---snip---

So, if I interpret these statements correctly, explicitly specifying the
background color as "rgbt 1" might do the trick.

Did you already try adding a "#version 3.6" statement to your scene file?

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