Yes it does once the jackpot reaches roughly 160 million the takings from ticket sales is no longer added to the jackpot and is instead added to the next tier down.
check this page 5+1 star prize pool was circa 6 million the jackpot was 161,653,000
https://www.national-lottery.co.uk/...zeBreakdown.ftl?drawNumber=397&drawSequence=0
2 draws previous before it reached the cap the prize pool was only 1 million yet the jackpot was 159 million
https://www.national-lottery.co.uk/...zeBreakdown.ftl?drawNumber=395&drawSequence=0
It's just the way they have to do it by law as a certain percentage has to go back into the winnings, once the cap is reached they have to drop this to the next tier if that tier reaches it's cap then it goes to the next tier again and so on.
usually tho on a capped jackpot all tiers have some increase on winnings but 5+1 is the most significant