When Minecart is used on rails, its behaviour is as intended until it comes across a unpowered powered rail such that whatever speed the Minecart will be going at, it will come to a standstill. In older version, when Minecart used to go through powered rail, the powered rail if not being powered would just act as normal rail and not increase or decrease speed of Minecart, but now it just stops Minecart on the spot itself.

The following OneDrive link contains a video which shows two rail tracks with same powered rail configuration, one on right will have all powered rails powered but the one on left will have one powered rail unpowered:

https://1drv.ms/v/c/1df1358942aca722/EXyfgKaHwklLp91GxuHW4A4BVnX5JmibzoyOPbptMK5qqQ?e=MpipdR