Author here: I always wash my clothes in cold water, 100% of the time. I really don't know what happened, but the T-shirt was already ruined after 3 or 4 washes. Any advice is more than welcome :)
Basically try to not wash your merino and if you do, handwash.
Otherwise just try to get merino blends which will hold up a bit better.
Noted, thank you :)
Merino is a breed of sheep. Sheep produce wool. You are not supposed to wash wool frequently, nor in machine washers.