If you want to get to the root cause of the issue the engine needs to be disassembled (crank & rods out). Then the seal retaining groove must be measured for concentricity to the main bearing bore, also checking the rear seal retainer.

The main bearing bores can be crooked (seen this with the naked eye on a SBC once ) or the seal retaining groove can be machined offset from the centerline of the main bores. Don't know if there is anything to correct the crank being crooked in the block but the seal retaining groove I'd assume could be corrected by machining and making a custom insert for the seal.

If everything is out of spec in that area there is always a rope seal but that is not always rainbows and unicorns either.

The crankshaft also needs to be checked for proper surface finish and size.

Could also be the fact whatever seal was used has inherent issues in quality so the seal itself needs to be inspected as well. Here you better know exactly what your looking for when determining the quality of the seal. Due to the fact a major seal manufacturer has quit production on a 2 piece rear main seals parts suppliers are scrambling for whatever they can put into their kits and keep them going out the door.

Last but not least, installation errors. Common, very common. I don't care if someone has been assembling engines for 30-40 years, they can and I have seen them doing it wrong for 30-40 years yet claim they have never had a problem.

I was recently (10/20) called in to a local business who was in panic mode over a rear main seal leak on a 440 dyno mule. There were 4 different people who put 8 different seals and seal retainers in trying to get a leak to stop but every time it went on the dyno it would sling oil up the walls
I grabbed a seal and some gaskets off my shelf, headed over and tore into it. Installed my seal kit with the stock retainer, ZERO SEALING AIDS, only assembly lube. Put the pan gasket and oil pan on, rolled it over to the dyno, hooked it up, fired it up and made 3 pulls. Wouldn't you know it, DRY AS A BONE!

One of the guys was so skeptical he had to take my picture while harassing me about my soon to be failure
As of 11/17 it's still dry after many dyno beatings testing 2 sets of heads and 4 different cams