Speed and sync issues

It very well could be me, but when I am using the app on some songs, I am finding that they "seem to’ speed up or slow down or go out of sync.

I played today Good Riddance on the app, and I know the song pretty well, I had found it hard to keep it going or in time even with just 4 down strums.

I have other songs where I have no issues, ie: Lips are moving I seem to be doing well.

It could be that Good Riddence doesn’t fit a basic strumming pattern as nicely as the fast one it already has, and my ears need to get used to that, but I wonder if anyone else had this issue?

I also feel like the singing is way out of time. I had to turn it off and just use the band.