Seller is suddenly making me concerned

Hi all, FTB here currently in the middle of conveyancing. We are a family of 4 with 2 very young children buying a semi-detached house about 35 miles north of where we currently rent as buying in the commuter belt round London is near-impossible. We found this place, its near good primary and secondary schools, and close enough for me to walk to the train station for the days I have to be in London. The sellers had a chain collapse on them previously, and dropped their asking price by £30k based on work that the house needs (new boiler and windows), they accepted our offer and all has been going well.

Until the survey.

Everything in the house was noted as a red 3, and speaking to the surveyor he said that it is in the bottom 25% of houses he has surveyed due to "bodge jobs and work half finished and just generally not being looked after". One particular item of note was that the rear ventilation bricks had been covered and the paving installed and covering them was almost at the level of the damp-proof membrane. He recommended having someone check the sub-floor joists to ensure no rot. I asked the seller through the EA about this and they have said that we're more than welcome to have tradies come in and estimate for work/check the house, but not until after exchange of contracts.

To me this seems a huge red flag as it suggests that there is something to hide, but Im also aware Im a FTB and dont have experience, so is this a normal thing, or am I right to be concerned with their behaviour?

Thanks in advance.