I think there are actually two distinct factors going on here.

The first is that the traditional rightward shift as you age has broken down among millennials, as you note.

The second - and I actually think this is as if not more important - is that the Tories have abandoned the field on left/right 'economic-based' politics anyway. Sunak presided over the highest tax burden in 70 years. The Tories' post-2016 pitch to the electorate has always been about cultural conservativism - Brexit, immigrants, toilets for trans people, etc - not right-wing economics. And unlike left/right issues, there was never a trend for people to become more culturally conservative as they age. People just form their cultural norms and values when they're young, and then carry these values with them through life, reacting against things that diverge from their norms.

By abandoning economics for culture wars, the Tories have built their electoral castle out of demographic sand. As the people who grew up in an overwhelming white and insular 1950s and 60s Britain give way to Millennials and Gen Zs who grew up in a ethnically diverse EU member state, the Tories have increasingly set themselves up in opposition to the cultural norms of the British electorate - and that is a stench it's going to be hard for them to shift.