There was an API update to the Slideshow module late last year or early this year, which may explain why it worked for you in 2024.
Have you tried exporting the slideshow to video with the music disabled?
Are you combining still photos and videos into a single slideshow?
Have you tried resetting the LrC preferences? If not, then try Methos 1 as set out in https://helpx.adobe.com/lightroom-classic/help/reset-preferences.html
Can you provide any information on how others can try to reproduce the issue. For example, no mention of how many photos are in slideshow, the template used, the settings used in the Playback panel, etc.
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