On Thu, 18 Jul 2019 02:38:36 +0100, RJH wrote:

Moisture that side of the wall can evaporate. On the inside, warm air
hits cold wall, and the moisture in the air condenses. Ventilation may
be an issue too. The spores may have got into the plaster, and that part
of the wall is locked in a cycle of mould.

Sorry, you misunderstood but it's my fault having re-read what I wrote.
When I said the "opposite" side of the wall I meant the mirror image as
it were; the other *internal* wall side.