Created attachment 143563 [details]
The attached PPTX, created in PowerPoint contains a single slide piece of text in a text box, with Text Fill (Drawing Tools / Format tab) set to No Fill is invisible in PowerPoint.
In Impress the text is black (and after rountrip, in PowerPoint as well).
Not sure what can be done in Impress, setting color to white doesn't do it, as the text has to be invisible (ie. with any background).
Observed using LO 220.127.116.11 & 3.3.0 / Windows 7.
Created attachment 143564 [details]
Apparently what happens in the case of PPT is that such text is removed, see the attached sample PPT saved from the previous PPTX in PowerPoint.
I can confirm with Version: 18.104.22.168.alpha0+
Build ID: d11313f341d1d9a9c73c736932441a89d5292e37
CPU threads: 4; OS: Linux 4.4; UI render: default; VCL: gtk3;