Uk rail is in a bit of a mess at the moment.
The Covid pandemic has caused various issues, direct staff absences due to covid are one but far from the only one. A lot of driver/gaurd training was disrupted due to Covid measures. The Pandemic has also made the railway even more of a financial hole for the government and they are trying to tighten the belt leading to relationships between the operators and their staff and unions becoming more strained than usual.
The need to re-plan services to fit the staff available has also sucked up train planning resources, this has lead to timetables being confirmed and seat reservations released much later than they should be.
During the height of the pandemic most of the intercity operators made seat reservations compulsory, at least on paper. Currently my understanding is that none of the operators (except perhaps Caledonian sleeper who had compulsory reservations even pre-pandemic) are actually enforcing compulsory reservations on the ground, but some operators* are still marking their trains as "reservations compulsary" in the timetable data. Online retail systems (and an increasing proportion of ticket machines) won't sell you a ticket without a valid itinerary and that means if a train is marked as "reservations compulsory" they won't sell you a ticket against that itinerary unless they can obtain a reservation.
The data that feeds into the Retail systems cannot distinguish between a compulsory reservation train where all reservations have sold out, and one where the reservations have not been released yet.
National rail enquiries has a section for future engineering works, looking up your date I see there is disruption to Avanti services in the Preston area but nothing that would directly affect a Manchester to Milton keynes journey. There may well be indirect affects though.
This doesn't mean you can't travel, but it does mean if you want to be somewhere by a particular time then you should probably make a larger allowance than usual for delays and cancellations.
I don't use the trainline app, but normally re-booking is not actually necessary with UK rail tickets. If your train is cancelled you can just take a later one (subject to the operator and/or route restrictions on your ticket).
* It looks like GWR, Crosscountry, Grand Central and Hull trains have dropped the compulsory reservations flag. But Avanti, LNER and LUMO are still setting it.