I'm not really worried, nor am I jumping on the "Doctor Who is getting cancelled" train.
The show is streaming on iPlayer and Disney+, so there is a good chance that the vaunted 25-and-below segment aren't too hung up on flow TV air dates. They can watch it anytime, and once the word of mouth about "Lux" spreads they probably will.
ETA: This update on combined over-the-air and streaming ratings for "The robot revolution" suggest just this:
The Robot Revolution’s +7 update reveals viewers are finally getting the hang of early morning iPlayer […] the reveal of last week’s [viewing figures] now makes this overnight an effectively meaningless measure. In recent years, overnights had been little more than signposts signalling the general direction of the measures that matter. But if this pattern holds true, then the overnight rating will no longer give us anything like the full number of people watching the day of broadcast.