In message ,
Tricky Dicky writes
The plasterboard will dry but if it has swollen as you imply it will
not shrink back. The only real cure is to replace the damaged section
of PB. I would make sure that the tiles are actually still firmly glued
to the PB and not just wedged in place by the grout. If a repair is not
possible and matching the tiles not possible as long as the tiles are
secure you may just have to put up with some wider than ideal grout lines.

Excellent, thanks. Yes, I realise the PB will not shrink back, and yes,
the tiles seem very firmly attached. The cracks are at the 90 degree
outside corners of a recess, so happy to scrape off where cracked then
refill, even though the join line will be wider than normal.
--
Graeme