As we have noted many times on this forum, if owners corporations allow owners to make changes to common property without approval, then future owners corps inherit the duty to maintain and repair the changes adter the unit has been sold on to subsequent owners.
It sounds very much like that’s what’s happened here but there are a few options. The owners corp could agree to just “suck it up” and pay for the repairs.
Or you could say, that since this structure is common property and it is costing you money to repair it, you are just going to remove it.
Or you could agree with the current owners to repair this once, provided they agree to a by-law passing responsibility for all future repairs to them and whoever buys the unit in the future.
The current owners can’t have it both ways. If they insist it’s common property, then (unless I am seriously mistaken) you can decide whether or not to keep it in place.
There is a compromise to be had here – there are a lot of grey areas to be looked at.