I was ridding my minecarts and decided I wanted to see my behind the scenes Redstone but then get back in so I did /tick rate 10 did slow the minecart did /tick rate 50 sped it up did /tick rate 10 then /tick freeze the minecart didn't freeze everything else did though, you can then do /tick rate 50 and speed the Minecraft up. Here is video documentation, https://youtu.be/LhGaiwldMvY
This behavior is Working as Intended. Mounts the player is riding are unaffected by /tick freeze, per the release article: