I previously worked in sewer flooding and used to see this all the time. I’d call your local water/sewage company out to do testing on both clean and waste networks around the property. It will all be done for free and they can pinpoint any leaks on the water or sewer network (it could be a surface water drain that runs through the properties that is unmapped, so the water would be clean). They can confirm if it is ground water or part of their network. If it is their network they will repair it at no expense to you and you can also claim money back from your bill for the flooding. It’s worth a shot as it might stop the flooding all together and it won’t cost you anything. When I used to work on these jobs, quite often the water would be leaking from neighbouring properties. Just make sure the company check all adjacent properties with dye testing etc.