I would say that is not just true for teachers - it is also true for those working with a child 1:1, especially when this is just part of their duties.
Even when a child does have a 'full time' 1:1 (funding for this is rare), then another member of staff will be responsible for the child for some periods of time each day, to allow the main 1:1 to have breaks. This other member of staff may be coming from a child with part-time 1:1, from a class TA role, from another role in the school such as lunchtime supervision - and may be bringing all the mental overload from all of those responsibilities to that short time with the child.
Equally, many 1:1 arrangements are part time - as I said, full time 1:1 funding is rare - so again the person delivering this 1:1 may well be juggling their knowledge of the needs of the child with all the other things that make up another role within the school which fills the rest of their contracted hours.
That is before you get sickness, training or other absence cover into the mix.