The slowest speed I could notice this on is maxSpeed 40. It could be a bit lower but it's impossible to see due to minecart position issues (MC-275565)

Reproduction Steps

• Build this setup (world download: MC-275628 Repo.zip)
  
• Set minecartMaxSpeed to 100
• Close the loop and wait for the minecart to speed up (use /tick sprint)
• Change the rail shape with /setblock

Observed Result

• The minecart derails and flies through the air

Expected Result

• The minecart wouldn't derail since the track is continuous