I had a leak in the same location and bought a replacement radiator from eBay. The repair center that changed the oil pan gasket said I needed a new radiator to fix the leak. I had everything apart and discovered the ebay unit didn't have the hole for the transmission oil cooler.

There is a rubber o-ring that seals the oil cooler to the radiator. The leak was coming from there. I replaced the o-ring with a new one and no leaking since. I installed it in May 2023. Sent the radiator back and got a full refund.

This is the o-ring I got off Amazon. I went with Viton because of chemical and temperature compatibility. I had a butyl o-ring but Viton has a higher temperature rating. Paid $17/5 pcs

M4x38 Viton O-Ring, 75A Durometer, Round, Black, Viton, 38 mm ID, 46 mm OD, 4 mm Width

If you're talking about PN 17101439140, there are 2 of them and I've replaced them twice now. That area is clean, it looks like it's seeping between the radiator and mounting plate. I'm just gonna go ahead and replace the radiator and mounting plate with trans thermostat I guess.

The only thing I can advise is to do a cooling overhaul using ONLY BMW brand when
it comes to the mounting plate, reservoir + cap, hoses x 2.

Tstat: Wahler is good.
Rad: BMW vs Nissens.

Pulleys: INA.

Belts: Conti.

I got everything replaced the other day. Not too bad of a job. I didn't even pull her into the shop. So far no leaks! I couldn't find the smoking gun on the old radiator and mounting bracket but my suspicion is an old/dry o-ring. You can see in that one picture, there is one connection on the radiator and plastic mounting bracket. The bracket has an o-ring inside so technically it could be replaced if you found the right size, then you wouldn't need the $50 mounting bracket.

Parts replaced:
Radiator
Radiator mounting bracket
Coolant fitting
Trans thermostat
Trans cooler seals