probably inevitable with the ongoing rolling stock shortage:
Due to long term train cancellations, the following temporary service arrangements will apply from 22 May 2023 and supersede Working Timetable 18, until further notice. 5 trains have been withdrawn from the weekday timetable of WTT18. This is comprised of 2 trains which operate in the peak service only and 3 trains which operate throughout the traffic day. Existing trains have been rescheduled to minimise the impact of wide intervals that have opened up from the cancelled train paths. However some wider than usual intervals will remain in certain parts of the timetable, due to pathing constraints. Intervals are to be regulated and services adjusted where necessary.
There are multiple reasons why they can't just buy more trains for the Jubilee line - 1. They aren't made any more and all the tooling to make them will have been discarded and the machines put to use making much newer trains 2. TfL doesn't have the money to buy new trains at the moment 3. You can't just buy tube-gauge trains - they are a bespoke product designed and/or customised for each line so new trains would need to be designed, developed, built, tested and approved - this would take 4-5 years at absolute minimum, and more than likely longer, even if they had the money in hand 4. Any new trains that have different door spacing to the 1996 stock would require a complex and expensive programme of replacing the platform edge doors. The new trains being built for the Piccadilly line have different door spacing.
The essential things in life are seen not with the eyes, but with the heart. --Antoine de St. Exupery
An earlier plan for a build of 27 modern day equivalent trains, 10 for the Jubilee and 17 for the Northern Line, avoiding problems with platform edge doors, training and maintenance etc, finally came to an end in 2017 when tenders exceeded budget provision.