As commented by Henri Timmermans
Topic can be closed/changed to question. Then his answer as Best Answer.
Currently I don't think it's possible to change the order of the field in the 'Update' screen. It is possible that fields are ordered almost randomly. It doesn't use the List order so for an user it's very confusing. Here an example of a confusing order. I would like to be able to order this Update screen or at least it to be equal to the List.
Best answer by htimmermans
Just tried it in SF 2018.3.