Are you saying that the portal is taking data from OSM, or that the data in OSM is from an undocumented import of data from the portal (or some similar site)?
I’d suggest just reverting the initial import, but not deleting those where actual survey data has been added (extra tags, but not counting “reformatting phone numbers” as “extra data”).
I’m curious about how we know the copying is in that direction - do sites get updated on the portal when changed in OSM?
Or could both OSM and that portal have imported data from a 3rd source like a tourism directory? I note the portal has information about facilities like swimming pools that doesn’t seem to have been in OSM before that site was deleted.
I suspect that there is a bit of citogenesis going on - something got added to OSM from some unclear source, but once there there is lots of information to “back it up”.