Just a theory:

There was a Poser bug that spontaneously changed the gamma value associated with certain images. When this happened to transparency, it made them appear very weak - not opaque enough. If you were experiencing that bug you may have compensated, through various adjustments, to make the opacity higher. If, following the cessation of said bug, you were to observe the same scene, it would render very dense given your compensations to the now-missing bug.

Perhaps SR5 "fixed" it?

Also - for all I know, the bug is still with us and is just choosing different moments to manifest itself, perhaps even going so far as to do the opposite of what it used to do, and make things more opaque instead of less.

We'd need lots more info and verified (not assumed) gamma values before and after to prove it.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)