Can anyone identify what's happening here? It's an old solid brick wall - don't know what the paint is but it looks like blistering from trapped moisture (although more extreme than other examples I've seen hence not being 100% sure)

Possibly efflorescence caused by salt migration. Usually white but can be other colours depending on the type of salt. Can look unsightly which is possibly why they painted the wall.

Appears frequently on new houses in the first few years or when water levels change. Lasts until all the salt in the bricks is washed out naturally. Pressure washing can make it worse. Sometimes it can be brushed off. For new builds allways check if the bricks you plan to use are considered "Low efflorescence".

On older houses you would hope all the salts had been washed out but perhaps the water table has changed (new concrete floor or patio? Leaking drains or gutters?

Have soil levels changed the other side?

Thanks both - more or less as I thought just never seen it this pronounced through paint.

Sandblasting paint off historic brickwork sounds a risky strategy though. Have you done that yourself Tony?

Yes I have had paint sand blasted off brickwork, the bricks were relatively hard compared to the mortar and the contractor had a wide range of different sands to choose from, with soft bricks it could be much more difficult. I did once dream of using a needle gun to get paint off bricks but never did try it.