You are doing the work so you should take responsibility for that work.
The OC has no control over it – look at it this way – you could renovate the bathroom including waterproofing, the job is really bad. At a point in time in a few years it is necessary to rip out the bathroom and fix the waterproofing – if the OC were responsible for it, then in theory they would also be responsible for replacing the entire bathroom. I don’t think so.
It is not clear to me why a time limit has been specified for the responsibility being on the owner. If it were me, the responsibility would rest with the owner in perpetuity.