I’ve been facing this frustrating problem. Apparently when any animation is playing other than the plain Appear animation, any object that has transparency in the slide will break. It will either disappear, or lose its transparency. This applies for PNG images, as well as shapes you draw directly within the Impress.
I have included a link to the video demonstrating this problem: https://youtu.be/XaRDMYs11Aw
My LibreOffice version:
Version: 26.2.3.2 (X86_64)
Build ID: 70e089b17412e4cb7773e41413306b17a2328c34
CPU threads: 16; OS: Windows 11 X86_64 (build 26220); UI render: Skia/Raster; VCL: win
Locale: en-US (en_US); UI: en-US
Calc: CL threaded
Note: I have tried disabling Skia and enabling Hardware Acceleration, this apparently has no effect in LibreOffice 26.2.3.2 as well as LibreOffice 25.8.5.1, but it does fix the problem in LibreOffice_7.6.7.2. Which suggests that the problem lies somewhere with Skia rendering. I would downgrade to 7.6.7.2 but it becomes painfully slow without Skia and has other bugs.
Transparency_Breaking_in_animation.odp (59.1 KB)