I’m a social worker, not a doctor, but to attempt to answer your question, I looked on PubMed, the government site where many journal articles can be found. I found an article from 2010 that lists several possible causes of this problem as well as some ways to prevent and treat it. The full article is 16 pages long. I’d suggest you share the following link with your kidney doctor. The article is written for medical personnel, not patients, but if you read it, it may give you some ideas of questions to ask your kidney doctor.
Beth has shared a good to read research paper about it in my understanding.
I am also not a doctor but as an attendant to the patient ,i can correlate with your case that any your blood pressure systolic (upper number / remains high during dialysis such as 260 .
I am more curious to know how it’s measures (via arm cuff or legs ) and what is done to lower it down during the dialysis and are you facing any other symptoms with rising BP such as headache ,uneasiness etc .
Also , I assume you are doing in center Hemodialysis of yes then what are the the other vitals such as pulse rate(heart beat) ,SPo2 ?
I have seen all variance of BP at dialysis unit in hospital - with some patients BP remains on the lower side 95 to 130/ 55 to 65 while with some it goes to higher ranges such as 95-115/180-250 and in some cases no discomfortong symptoms at all. As per the research paper attached above if heart function is involved in it then you should rule out by doing 2D echo test. Why i suggest so because my wife is undergoing Dialysis and she was always having diastolic number (lower number of BP) on lower side 55 to 70 which made me in doubt so I asked doctor about this concern .Doctor asked me to get 2D echo for her done. Result came out as Low grade Diastolic dysfunction . As per doctor it is not a major problem to worry about.
I am guessing for some there could be systolic dysfunction. (Upper number )