I've never bookmarked anything but 8 / 10 times when I upvote something, load a different post and come back to the feed I was previously on the upvote will have disappeared.
Usually have to upvote most things 2 or 3 times before it shows as staying on my end.
I have this too, I figure it's just a visual / cache related bug. I don't bother reupvoting, if I come back later or stop and restart the app it's there.
It even happens with the same post, like if you upvote from the feed and then open the post it doesn't show as upvoted.
I had considered it could just be visual and was applying the upvote even if it was no longer showing on my end but I hadn't actually tested it. Good to know it actually does stay though.
I think it might just be a visual bug. When I upvote somethin,g then bookmark the upvote disappears, but when I open the comments the upvote is still there