The Office of Fair Trading (OFT) and your plumber are correct, and your Strata Manager is “splitting straws”.
All original tiles on the floor and on the external wall to which your refer are Common Property, and your Owners Corporation has an absolute responsibility to undertake all maintenance and repairs, and NOT to wait until the consequences of its inaction affects other Units.
The solution is to again advise your Strata Manager, this time in writing, that the areas requiring maintenance / repairs are Common Property, that the consequential affects of the fault is spreading, and that as concession on your part you will pay for the cost of the re-grouting of those areas post the repairs (those $ will be minimal). Add that if you do not receive an agreement to that within say 7 days, an Application for Mediation will be lodged with the OFT.
Hopefully you won’t need it, but that Application can be downloaded here https://www.fairtrading.nsw.gov…..edform.pdf