UPS is the short answer. UPS and a combination of general logistics things being all over the place courtesy of Brexit and COVID being the longer answer. Other couriers have demostrated far less issues, hence UPS footing the bulk of the blame.
Honestly, there isn't enough information to support how long this'll be the case. Being able to use a courier that isn't UPS would likely help, but TG have said multiple times that they aren't looking to change... or rather that they don't have the option to. Though I don't at all believe that UPS is the only courier they could get a contract with.
I'm sure there are folks that have had much less of an issue, but purely down to luck.