Northern English dialects typically use a short vowel /æ/ for "a" before an "s" or "th" where southern English dialects would use a longer, further back vowel /ɑ/. However there seem to be exceptions, of which "master" and "plaster" stand out for often having a southern English pronunciation. "master" is explicable as a word associated with education and authority, which therefore is sometimes pronounced in a sort of imitation of Received Pronunciation or other higher-status dialects, but "plaster" seems a bit of a mystery.
The British Library has a page on dialects which notes this. It says the pronunciation of "master" and "plaster" varies in the North of England, unlike in other words ("past", "blast", "cast", "castor", etc) which have a short vowel.
[ah is used in "plaster" and "caster" by] speakers in the north – notably on Tyneside and in Yorkshire and Lancashire – although these speakers favour 'a’ in all other instances. This gives rise to the intriguing combination of vowel sounds in words such as plastercast (‘plahstercast’) and elastoplast (‘elastoplahst’) and the Stevie Wonder song 'Master Blaster' becomes ‘mahsterblaster'. ‘Mahster’ and ‘plahster’ do not seem to occur on Merseyside, in Manchester nor in the flat-BATH areas of the Midlands, where pronunciation of the TRAP~BATH sets is consistently ‘a’.
The BBC Voices project, which records dialects across the UK, notes:
There are, however, two words that many northerners pronounce with a 'southern' long vowel: master and plaster. It's possible that the former has undergone change as a result of its association with school, education and notions of 'prestige' pronunciation, although the latter is harder to explain.
Sources don't discuss specific reasons for a generational change in "master" in the north of England. But pronunciations are constantly shifting, and you would expect a regularisation of pronunciations so "master" would tend to be pronounced like "castor", and all the other words with "-ast", which is what the OP describes. (Annoyingly the recorded voices on the BBC website don't work for me.)