Arthur had lost weight but he’s healthy. Disappointing about the weight.

Horses lose weight a lot more easily than they put on weight. It takes much longer time frame to build up from weight loss than it does to lose the weight.
Horses don't cope well with feed changes. They don't cope well with being starved or given feed after being starved. It can cause colic and colic is most horse owners' worst nightmare, as it can come on suddenly and kill horses very quickly.
The horses went for 5 days in a row without feed and then another day without feed. There was nothing we could do about it. It involved legal action to ensure we could get feed in and the RSPCA had closed off their call centre, due to COVID lockdown. Not sure they could have done anything, anyway.