Where’s your video located in the title explorer? Is it on chapter or title level?
I just tested this in Lectora 17.1.4:
My video is on chapter level.
When I open the first page the caption buttons id is “mep_0_captions_none”
When I go to the next page that also contains the video from chapter level the id is “mep_1_captions_none”
When I return to the first page the id is “mep_2_captions_none”
So it may not be able to find the button by id because it does not exist.
When published without Seamless Play the id stays the same “mep_0_captions_none”