Unusual stem?

Yes it does happen with Primulas two, or more, stems fuse together and you get this huge head of flowers - it doesn't mean that the plant is diseased or anything

Isn't this condition known as Fascination? Is the stem flat?

Fasciation, close greenfingers.
Thanks for that I've just looked it up.
Abnormal plant growth in which several stems become flattened and fused together. Often associated with rapid growth.

I seem to remember a Forsythia suffering from this at my old house in Oxford. But that was before I was in to gardening.

I was going to say 'fascination' knowing that the word was not quite correct. I have seen it a few times with primula and this year had it with a Fritillaria.

I've seen it on foxgloves and veronicastrum as well

Yes, and other plants as well - a Leucanthemum I think.

its often caused by damage to the apical meristem [growing tip] either by sap suckers or frost apparently. so i suspect we will see a lot of it this year after the winter we have had.