Flatmates are moving out, estate agents state that my entire contract terms I agreed to are void due to them leaving? (England, East Riding of Yorkshire)

The current contract states that the term is "A fixed term from and including 11/03/25 until and including 10/03/26 and then continuing on a monthly contractual periodic basis thereafter".

I was planning to use the remaining time (up until March 10th, when this contracted ends) to move to a different part of the UK, but if necessary taking advantage of the month-by-month contract terms if I happened to need extra time.

My sister and her boyfriend were the other tenants, she has bought a house in the same location we currently live and is finally in a position to move in to it after painting etc is finished.

The estate agents claim that because they are leaving, it voids the entirety of the contract and that I must sign a new, 6 month minimum term tenancy agreement with none of the current contracts' terms, also with no guarantee of a break clause implemented (they are currently trying to find out from my landlord of that is even "agreeable").

After a few months of living here, a new person moved in below us (there is 2 flats here total) and she turned out to be a drug dealer, she regularly had rough/dangerous people visiting for drug deals, also partying until 4am multiple times a week and with all of that came visits from the police, trying to get it resolved we cooperated with them and also the landlords as much as possible. This individual also accused me of taking pictures of her and stalking, she harrassed my landlord from various numbers to attempt to get my name - presumably so the type of people she was involved with could harm me. Of course none of this was true, I am also in a committed long term relationship, which is long distance (hence me moving elsewhere by march).. The whole ordeal caused me a great deal of anxiety, (I had GAD diagnosed 2 years ago) and left me dreading leaving my flat for 5 months out of fear of being harmed.

The landlords are also currently wanting to sell the property, and on numerous occasions I have allowed someone to view the property of short notice (less than 24hrs notice) out of courtesy to the landlords.

Everything considered they seem to have no care for everything both sides have had to deal with this year, or any favours we allowed them.

Is this legal? If so, surely it is at least really harsh and punishing. Do I have any grounds to stand on and fight this? All i'm asking for is a little bit of leeway and to honour the original contract terms - they still get their full amount of money each month.