Not disagreeing on this because the US is very much its own thing, but the UK also uses FPTP and the collapse of mainstream (LibLabCon) party vote has been quite stark in recent years, and FPTP becomes hugely unstable when the vote becomes split.
For context, Plaid is widely tipped to become the biggest party in Wales at the next GE and the SNP kicked the mainstream parties out of Scotland a number of elections ago. The Greens and Reform are likely to take England.