Given they managed to swap an HST from an Aberdeen to Inverness to work 1B30 1248 to Edinburgh so it could go HST as booked and the 1733 Edinburgh to Inverness instead of both bring a 158 (which went on the Inverness to Aberdeen instead), they do if they can in my experience.
I think Aberdeen was already short of HSTs/170s from yesterday based on the formations I saw on RTT, hence the morning turn that forms 1B30 off Inverness being a 158 instead of HST; so presumably there was nothing else bar the 158 to go to Glasgow.
Perhaps I'm wrong, but there always seems to be more substitutions on a Monday currently than the other weekdays, presumably because it follows Sunday and Sunday is one of the worst days for general operations at the moment, so various services get subbed on Suns, so the relevant trains aren't all in place for Monday, so Monday has several substitutions etc etc
That was my educated guesswork anyway...