So, you create a Nether Portal in the Overworld, step through into the Nether and do your thing. Step back through the same Nether Portal you just came out of and a NEW Nether Portal is created in the Overworld. It is impossible to return to the first Nether Portal.

I’m not talking about multiple portals, or anything like that, just one on the Overworld and one in the Nether.

Very frustrating

See the attached clip - it's very strange!