Created attachment 174992 [details]
I have a PPTX that has been saved from PowerPoint 2010.
It should come with two master slides as described in the screenshot attached.
On libreoffice 7.2 it doesn't provide any master slide.
Created attachment 174993 [details]
Master slide selection in PowerPoint 2010
Version: 188.8.131.52.alpha0+ (x64) / LibreOffice Community
Build ID: 8151f3a1d99ab740d2affdccc7115faa156bf3ad
CPU threads: 4; OS: Windows 6.1 Service Pack 1 Build 7601; UI render: Skia/Raster; VCL: win
Locale: ru-RU (ru_RU); UI: ru-RU
Created attachment 175018 [details]
The example file in PP 2019 and Impress master
These are not master slides, but custom layouts, which are not used on any slides.
Creating custom layouts is not supported in Impress (UI-wise), so it's not surprising these are not imported.